U.S. patent application number 12/134832 was filed with the patent office on 2009-06-11 for system, apparatus, and method for measuring an ion concentration of a measured fluid.
Invention is credited to Klaus K. Allmendinger.
Application Number | 20090145778 12/134832 |
Document ID | / |
Family ID | 40720503 |
Filed Date | 2009-06-11 |
United States Patent
Application |
20090145778 |
Kind Code |
A1 |
Allmendinger; Klaus K. |
June 11, 2009 |
System, Apparatus, And Method For Measuring An Ion Concentration Of
A Measured Fluid
Abstract
An apparatus, system and method maximizes efficiency and
accuracy of measuring an ion concentration of a measured fluid by
varying a flow of ions within a measuring cell (1006) in accordance
with an output signal of a sensor cell. The pump current through a
pump cell is switched between a constant positive current and a
constant negative current when upper and lower thresholds of the
output signal are reached. The pulse width ratio of the square wave
produced by the varying current is compared to a pulse width ratio
function derived from a calibration procedure to determine the ion
concentration of the measured fluid. In one embodiment, the
functions of the pump cell and sensing cell are performed by a
single electrochemical cell. In an embodiment where a concentration
of a compound is determined, a primary electrochemical cell system
pumps an ion of an element of the compound into and out from a
measuring chamber. A secondary electrochemical cell system reduces
the compound to ions of the same element as pumped by the primary
electrochemical cell system to form a local concentration of the
ion near an electrode of the secondary electrochemical cell system.
The secondary electrochemical cell system pumps the ions into and
out from the measuring chamber in accordance with the relationship
between the general ion concentration measured by the primary
electrochemical cell system and the local ion concentration
measured by secondary electrochemical cell system. The duty cycle
of a secondary pump current flowing through the secondary
electrochemical cell system indicates the concentration of the
compound.
Inventors: |
Allmendinger; Klaus K.; (San
Juan Capistrano, CA) |
Correspondence
Address: |
CHARLES D. GAVRILOVICH, JR.,;GAVRILOVICH, DODD & LINDSEY, LLP
985 PASEO LA CRESTA, SUITE B
CHULA VISTA
CA
91910-6729
US
|
Family ID: |
40720503 |
Appl. No.: |
12/134832 |
Filed: |
June 6, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11767629 |
Jun 25, 2007 |
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12134832 |
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11244210 |
Oct 5, 2005 |
7249489 |
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11767629 |
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10699182 |
Nov 1, 2003 |
6978655 |
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11244210 |
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60443628 |
Jan 30, 2003 |
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60942781 |
Jun 8, 2007 |
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Current U.S.
Class: |
205/789 ;
204/406; 204/416 |
Current CPC
Class: |
G01N 27/419
20130101 |
Class at
Publication: |
205/789 ;
204/406; 204/416 |
International
Class: |
G01N 27/26 20060101
G01N027/26; G01N 27/333 20060101 G01N027/333 |
Claims
1. An apparatus comprising: an electrochemical cell for providing
an output signal in accordance with an ion concentration within a
measured fluid within a measuring chamber and for adjusting an ion
flow between the measuring chamber and ambient fluid in accordance
with a pump current flowing through the electrochemical cell; and a
current managing unit for varying the pump current between a first
constant current and a second constant current in accordance with
the output signal.
2. An apparatus in accordance with claim 1, wherein the
electrochemical cell comprises: an ion sensor cell for providing
the output signal in accordance with the ion concentration within
the measured fluid; and a pump cell for adjusting the ion flow
between the measuring chamber and ambient fluid in accordance with
the pump current flowing through the pump cell.
3. An apparatus in accordance with claim 2, further comprising: a
computing device configured to determine the ion concentration
based on a pulse width ratio of a square wave of the pump
current.
4. An apparatus in accordance with claim 3, wherein the current
managing unit is configured to vary the pump current by maintaining
the first constant current in a first direction until a first
output signal threshold is detected and maintaining the second
constant current in a second direction until a second output signal
threshold is detected.
5. An apparatus in accordance with claim 4, wherein the current
managing unit comprises: an analog comparator circuit configured to
provide a comparator output signal based on the output of the ion
sensor cell, the comparator output signal indicating when the first
output signal threshold is reached and when the second output
signal threshold is reached; and an inverting amplifier circuit
connected between the analog comparator circuit and the ion sensor
cell, the inverting amplifier circuit configured to change the
direction of the pump current in response to the comparator output
signal.
6. An apparatus in accordance with claim 5, wherein the computing
device is configured to determine the pulse width ratio by:
measuring a first time period corresponding to the first constant
current; measuring a second time period corresponding to the second
constant current; determining the pulse width ratio based on the
first time period and the second time period; and determining the
ion concentration by comparing the pulse width ratio to a pulse
width ratio function for the electrochemical cell.
7. An apparatus in accordance with claim 6, wherein the computing
device is connected to the comparator circuit, the computing device
configured to measure the first time period and the second time
period based on the comparator output signal.
8. An apparatus in accordance with claim 2, wherein the ion sensor
cell is a gas ion sensor cell and the measured fluid is a measured
gas.
9. An apparatus in accordance with claim 8, wherein the gas ion
sensor cell is an oxygen sensor cell and the ambient fluid is
ambient air.
10. An apparatus in accordance with claim 8, wherein the gas ion
sensor cell is a nitrogen sensor cell responsive to gaseous oxides
of nitrogen.
11. An apparatus configured to connect to a measuring cell, the
apparatus comprising: a current managing unit configured to receive
an output signal based on an ion concentration within a measured
fluid within a measuring chamber and configured to adjust an ion
flow between the measuring cell and ambient fluid by varying, in
accordance with the output signal, a pump current flowing through a
pump cell of the measuring cell between a first constant current
and a second constant current.
12. An apparatus in accordance with claim 11, wherein the output
signal is produced by an ion sensor cell of the measuring cell.
13. An apparatus in accordance with claim 12, wherein the ion
sensor cell is a gas ion sensor cell and the measured fluid is a
gas.
14. An apparatus in accordance with claim 13, wherein the gas ion
sensor cell is an oxygen sensor cell and the ambient fluid is
ambient air.
15. An apparatus in accordance with claim 13, wherein the gas ion
sensor cell is a nitrogen sensor cell responsive to gaseous oxides
of nitrogen.
16. An apparatus in accordance with claim 11, wherein the output
signal is a voltage across a single electrochemical cell and
wherein the pump current is through the single electrochemical
cell.
17. An apparatus in accordance with claim 11, further comprising: a
computing device configured to determine the ion concentration of
the measured fluid based on a pulse width ratio of a square wave of
the pump current.
18. An apparatus in accordance with claim 11, wherein the current
managing unit is configured to vary the pump current by maintaining
the first constant current in a first direction until a first
output signal threshold is detected and maintaining the second
constant current in a second direction until a second output signal
threshold is detected.
19. An apparatus in accordance with claim 18, wherein the computing
device is configured to determine the pulse width ratio by:
measuring a first time period corresponding to the first constant
current; measuring a second time period corresponding to the second
constant current; determining the pulse width ratio based on the
first time period and the second time period; and determining the
ion concentration of the measured fluid by comparing the pulse
width ratio to a pulse width ratio function for the measuring
cell.
20. An apparatus comprising: an interface configured to connect to
a measuring cell and receive an output signal based on an ion
concentration of a measured fluid within a measuring chamber, the
output signal produced by an ion sensor cell of the measuring cell;
and a current managing unit configured to adjust an ion flow
between the measuring chamber and ambient fluid by varying, in
accordance with the output signal, a pump current flowing through a
pump cell of the measuring cell between a first constant current
and a second constant current.
21. An apparatus in accordance with claim 20, wherein the current
managing unit is configured to vary the pump current by maintaining
the first constant current in a first direction until the output
signal reaches a first output signal threshold and maintaining the
second constant current in a second direction until the output
signal reaches a second output signal threshold, the apparatus
further comprising: a computing device configured to determine the
ion concentration of the fluid based on a pulse width ratio of a
square wave of the pump current resulting from the varying of the
pump current.
22. An apparatus in accordance with claim 21, wherein the ion
sensor cell is a gas ion sensor cell and the measured fluid is a
gas.
23. An apparatus in accordance with claim 22, wherein the gas ion
sensor cell is an oxygen sensor cell and the ambient fluid is
ambient air.
24. An apparatus in accordance with claim 22, wherein the gas ion
sensor cell is a nitrogen sensor cell responsive to gaseous oxides
of nitrogen.
25. An ion concentration measuring system comprising: a sensor
comprising: a measuring chamber having an ambient opening
configured to receive an ambient fluid and a measuring opening
configured to receive a measured fluid; and an electrochemical cell
configured to change an ion concentration within the measuring
chamber in accordance with a pump current flowing through the
electrochemical cell and configured to present a cell voltage in
accordance with the ion concentration; and a sensor measuring
device configured to vary the pump current between a first constant
current and a second constant current in accordance with the cell
voltage and configured to determine the ion concentration based on
the cell voltage.
26. An apparatus comprising: a primary electrochemical cell system
configured to generate a first output signal in accordance with a
first ion concentration within a measuring chamber and to vary a
first ion flow into the measuring chamber and out from the
measuring chamber in response to a primary pump current varying
between a first constant primary pump current and a second constant
primary pump current; and a secondary electrochemical cell system
configured to generate a second output signal in accordance with a
second ion concentration within the measuring chamber and to vary a
second ion flow into the measuring chamber and out from the
measuring chamber in response to a secondary pump current varying
between a first constant secondary pump current and a second
constant secondary pump current, the secondary pump current based
on a relationship between the first output signal and the second
output signal.
27. An apparatus in accordance with claim 26, wherein the first ion
concentration and the second ion concentration are concentrations
of an ion of an element, the first ion concentration within a first
region of the measuring chamber and the second ion concentration
within a second region within the measuring chamber.
28. An apparatus in accordance with claim 27, wherein the secondary
electrochemical cell system is further configured to reduce a
compound into the ion of the element and at least one other ion of
another element.
29. An apparatus in accordance with claim 28, wherein the first
electrochemical cell system comprises oxygen electrochemical cell
and the second electrochemical system comprises a nitrogen
sensitive electrochemical cell that reduces oxides of nitrogen into
oxygen ions and nitrogen ions at an electrode and wherein the
second region is closer to the electrode than the first region.
30. An apparatus in accordance with claim 26, wherein the first
electrochemical cell system comprises a pump cell and a measuring
cell.
31. An apparatus in accordance with claim 26, wherein the first
electrochemical cell system comprises a single electrochemical
cell.
32. An apparatus in accordance with claim 26, further comprising a
sensor managing device configured to vary the primary pump current
between the first constant primary pump current and the second
constant primary pump current and to vary the secondary pump
current between a first constant secondary pump current and a
second constant secondary pump current
33. An apparatus in accordance with claim 32, wherein the sensor
managing device is configured to: direct the primary pump current
in a positive direction at a first constant magnitude until the
first output signal reaches an upper threshold; direct the primary
pump current in a negative direction at the first constant
magnitude until the first output signal reaches a lower threshold;
direct the secondary pump current in a positive direction at a
second constant magnitude until a first difference between the
second output signal and the first output signal reaches a first
difference threshold; and direct the secondary pump current in a
negative direction at the second constant magnitude until a second
difference between the second output signal and the first output
signal reaches a second threshold difference.
34. An apparatus in accordance with claim 33, wherein the sensor
managing device is further configured to determine an ion
concentration of the at least one other ion of another element.
35. An apparatus in accordance with claim 34, wherein the sensor
managing device is further configured to determine the ion
concentration of the at least one other ion of another element
based on a duty cycle of a waveform in accordance with the
secondary pump current.
36. An apparatus in accordance with claim 35, wherein: the first
electrochemical cell system comprises oxygen electrochemical cell
and the second electrochemical system comprises a nitrogen
sensitive electrochemical cell that reduces oxides of nitrogen into
oxygen ions and nitrogen ions at an electrode; the second region is
closer to the electrode than the first region; and the sensor
managing device is further configured to determine a nitrogen ion
concentration based on a duty cycle of the secondary pump
current.
37. A method comprising: directing a primary pump current through a
primary electrochemical cell system between a first constant
primary pump current and a second constant primary pump current to
direct a first ion flow into a measuring chamber and out from the
measuring chamber; detecting a first output signal generated by the
primary electrochemical cell system in accordance with a first ion
concentration within a measuring chamber; detecting a second output
signal generated by a secondary electrochemical cell system in
accordance with a second ion concentration within the measuring
chamber; directing, based on a relationship between the first
output signal and the second output signal, a secondary pump
current through the secondary electrochemical cell system between a
first constant secondary pump current and a second constant
secondary pump current to direct a second ion flow into the
measuring chamber and out from the measuring chamber.
38. A method in accordance with claim 37, wherein the first ion
concentration and the second ion concentration are concentrations
of an ion of an element, the first ion concentration within a first
region of the measuring chamber and the second ion concentration
within a second region within the measuring chamber.
39. A method in accordance with claim 38, wherein the second ion
concentration is a ion concentration of an ion of an element of a
compound that is reduced into the ion of the element and at least
one other ion of another element at the secondary electrochemical
cell system.
40. A method in accordance with claim 39, wherein the first ion
concentration is a general oxygen ion concentration and the second
ion concentration is a local oxygen ion concentration closer to an
electrode than general oxygen ion concentration, the secondary
electrochemical cell system comprising a nitrogen sensitive
electrochemical cell having the electrode that reduces oxides of
nitrogen into oxygen ions and nitrogen ions at the electrode.
41. A method in accordance with claim 37, wherein directing the
primary current comprises directing the primary current through a
pump cell of the first electrochemical cell system and wherein
detecting the first output signal comprises detecting a measuring
cell output signal of a measuring cell of the primary
electrochemical cell system.
42. An apparatus in accordance with claim 37, wherein directing the
primary current comprises directing the primary current through
single electrochemical cell and wherein detecting the first output
signal comprises detecting a single cell output signal of the
single electrochemical cell.
43. A method in accordance with claim 39, further comprising:
directing the primary pump current in a positive direction at a
first constant magnitude until the first output signal reaches an
upper threshold; directing the primary pump current in a negative
direction at the first constant magnitude until the first output
signal reaches a lower threshold; directing the secondary pump
current in a positive direction at a second constant magnitude
until a first difference between the second output signal and the
first output signal reaches a first difference threshold; and
directing the secondary pump current in a negative direction at the
second constant magnitude until a second difference between the
second output signal and the first output signal reaches a second
threshold difference.
44. A method in accordance with claim 43, further comprising:
determining an ion concentration of the at least one other ion of
another element.
45. A method in accordance with claim 44, wherein the determining
the ion concentration comprises determining the ion concentration
of the at least one other ion of another element based on a duty
cycle of the secondary pump current.
46. A sensor managing device comprising: a first switching current
source configured to direct a primary pump current through a
primary electrochemical cell system between a first constant
primary pump current and a second constant primary pump current to
direct a first ion flow into a measuring chamber and out from the
measuring chamber; a first detection circuit configured to detect a
first output signal generated by the primary electrochemical cell
system in accordance with a first ion concentration within a
measuring chamber and to control the first switching current
source; a second detection circuit configured to detect a second
output signal generated by a secondary electrochemical cell system
in accordance with a second ion concentration within the measuring
chamber and generate a control signal based on a relationship
between the first output signal and the second output signal; and a
second switching current source configured to direct, in response
to the control signal, a secondary pump current through the
secondary electrochemical cell system between a first constant
secondary pump current and a second constant secondary pump current
to direct a second ion flow into the measuring chamber and out from
the measuring chamber.
47. A sensor managing device in accordance with claim 46, wherein
the first ion concentration and the second ion concentration are
concentrations of an ion of an element, the first ion concentration
within a first region of the measuring chamber and the second ion
concentration within a second region within the measuring
chamber.
48. A sensor managing device in accordance with claim 47, wherein
the second ion concentration is a ion concentration of an ion of an
element of a compound that is reduced into the ion of the element
and at least one other ion of another element at the secondary
electrochemical cell system.
49. A sensor managing device in accordance with claim 48, wherein
the first ion concentration is a general oxygen ion concentration
and the second ion concentration is a local oxygen ion
concentration closer to an electrode than general oxygen ion
concentration, the secondary electrochemical cell system comprising
a nitrogen sensitive electrochemical cell having the electrode that
reduces oxides of nitrogen into oxygen ions and nitrogen ions at
the electrode.
50. A sensor managing device in accordance with claim 46, wherein
the first switching current source is configured to direct the
primary current through a pump cell of the first electrochemical
cell system and wherein first detection circuit is configured to
detect a measuring cell output signal of a measuring cell of the
primary electrochemical cell system.
51. An sensor managing device in accordance with claim 46, wherein
the first switching current source is configured to direct the
primary current through a single electrochemical cell and wherein
the first detection circuit is configured to detect a single cell
output signal of the single electrochemical cell.
52. A sensor managing device in accordance with claim 48, wherein:
the first detection circuit comprises a first comparator configured
to compare the first output signal to a reference voltage and to
indicate when the first output signal has reached an upper
threshold and when the first output signal has reached a lower
threshold; the first switching current source is configured to
direct, in response to the first detection circuit, the primary
pump current in a positive direction at a first constant magnitude
until the first output signal reaches the upper threshold and to
direct the primary pump current in a negative direction at the
first constant magnitude until the first output signal reaches a
lower threshold; the second detection circuit comprises a second
comparator configured to compare the second output signal to at
least a component of the first output signal and to generate the
control signal in accordance with a difference between the second
output signals and the at least a component of the first output
signal; and the second switching current source configured to
direct the secondary pump current in a positive direction at a
second constant magnitude until the difference reaches a first
difference threshold and to direct the secondary pump current in a
negative direction at the second constant magnitude until the
difference reaches a second threshold difference.
53. A sensor managing device in accordance with claim 52, further
comprising a computing device configured to determine an ion
concentration of the at least one other ion of another element.
54. A sensor managing device in accordance with claim 53, wherein
the computing device is further configured to determine the ion
concentration of the at least one other ion of another element
based on a duty cycle of the secondary pump current.
55. An apparatus in accordance with claim 54, wherein: the first
electrochemical cell system comprises oxygen electrochemical cell
and the second electrochemical system comprises a nitrogen
sensitive electrochemical cell that reduces oxides of nitrogen into
oxygen ions and nitrogen ions at an electrode; the second region is
closer to the electrode than the first region; and the computing
device is further configured to determine a nitrogen ion
concentration based on the duty cycle of the secondary pump
current.
56. An apparatus comprising: an oxygen electrochemical cell system
configured to generate a first output signal in accordance with a
general oxygen ion concentration within a first region of a
measuring chamber and to vary a first oxygen ion flow into the
measuring chamber and out from the measuring chamber in response to
a oxygen cell pump current varying between a first constant oxygen
cell pump current and second constant oxygen cell pump current; and
a nitrogen electrochemical cell system configured to generate a
second output signal in accordance with a local oxygen ion
concentration within a second region of the measuring chamber and
to vary a second oxygen ion flow into the measuring chamber and out
from the measuring chamber in response to a nitrogen cell pump
current varying between a first constant nitrogen cell pump current
and a second constant nitrogen cell pump current, the nitrogen cell
pump current based on difference between the first output signal
and the second output signal.
57. A method of determining a concentration of oxides of nitrogen
(NOx), the method comprising: receiving, from an oxygen
electrochemical cell, a first output signal corresponding to a
first oxygen ion concentration within a measuring chamber; varying
a primary pump current through the oxygen electrochemical cell
between a first constant primary pump current and second constant
primary pump current; receiving from a nitrogen sensitive
electrochemical cell, a second output signal corresponding to a
second oxygen ion concentration within the measuring chamber, the
second oxygen ion concentration resulting from a reduction of NOx
into nitrogen ions and oxygen ions; varying a secondary pump
current through the nitrogen sensitive electrochemical cell between
a first constant secondary pump current and a second constant
secondary pump current in accordance with a relationship between
the first output signal and the second output signal; and
determining a NOx concentration based on a waveform of a signal in
accordance with the secondary pump current.
58. A method in accordance with claim 57, wherein the varying the
secondary pump current comprises: directing the first constant
secondary pump current through the nitrogen sensitive
electrochemical cell until a difference between the first output
signal and the second output signal reaches first threshold; and
directing the second constant secondary pump current through the
nitrogen sensitive electrochemical cell until the difference
between the first output signal and the second output signal
reaches a second threshold.
59. A method in accordance with claim 58, wherein: at least a
portion of the first output signal is an oxygen cell Nernst voltage
generated by the oxygen electrochemical cell in response to the
first oxygen ion concentration; and at least a portion of the
second output signal is an nitrogen cell Nernst voltage generated
by the nitrogen sensitive electrochemical cell in response to the
second oxygen ion concentration.
60. A sensor comprising: a measuring chamber configured to receive
a measured fluid having ions; an electrochemical measuring cell
configured to move ions between the measuring chamber and a sealed
chamber to produce a measuring chamber ion flow; an electrochemical
compensation cell configured to establish, in accordance with the
measuring chamber ion flow, a sealed chamber ion flow to move ions
into the sealed chamber and out of the sealed chamber.
61. A sensor in accordance with claim 60, wherein a first output
ion volume exiting the sealed chamber through the electrochemical
compensation cell corresponds to a first input ion volume entering
the sealed chamber through the electrochemical measuring cell and
wherein a second input ion volume entering the sealed chamber
through the electrochemical compensation cell corresponds to a
second output ion volume exiting the sealed chamber through the
electrochemical measuring cell.
62. A sensor in accordance with claim 61, wherein the measured
fluid is a gas.
63. A sensor in accordance with claim 62, wherein gas comprises
oxygen and the ions are oxygen ions.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part (CIP) patent
application of U.S. patent application Ser. No. 11/767,629 entitled
"System, Apparatus, And Method For Measuring An Ion Concentration
Of A Fluid" filed on Jun. 25, 2007 which is a continuation-in-part
(CIP) patent application of a U.S. patent application Ser. No.
11/244,210 entitled "System, Apparatus, And Method For Measuring An
Oxygen Concentration Of A Gas" filed on Oct. 5, 2005, now U.S. Pat.
No. 7,249,489 which is a divisional patent application of U.S.
patent application Ser. No. 10/699,182, filed on Nov. 1, 2003, now
U.S. Pat. No. 6,978,655, entitled "System, Apparatus, And Method
For Measuring An Oxygen Concentration Of A Gas" which claims the
benefit of priority of U.S. Provisional Application Ser. No.
60/443,628 filed on Jan. 30, 2003, entitled "System, Apparatus, And
Method For Measuring An Oxygen Concentration Of A Gas", all hereby
incorporated by reference in their entirety herein. This
application also claims the benefit of priority of U.S. provisional
application No. 60/942,781, entitled "Pulse Width Modulation
Wideband Ion Sensor", filed Jun. 8, 2007 and incorporated by
reference in its entirety herein. This application is also related
to International application number ______, entitled "System,
Apparatus, And Method For Measuring An Ion Concentration Of A
Fluid" filed on Jun. 6, 2008 and incorporated by reference in its
entirety herein.
BACKGROUND
[0002] The invention relates in general to ion sensors and more
specifically to an apparatus, system and method for monitoring an
ion concentration of a measured fluid.
[0003] Wideband ion sensors are used to measure the concentration
of particular ions within a fluid where the fluid may be a gas or
liquid. A popular use of wideband gas ion sensors includes using
oxygen sensors to determine on oxygen concentration within a gas
mixture. Other examples of gas ion sensors include nitrogen sensors
that sense gaseous oxides of nitrogen. Many conventional combustion
engines utilize oxygen sensors for determining the air to fuel
mixture of the exhaust of the combustion engine. Conventional
internal combustion engines typically incorporate electronic
fueling control using computing devices, such as Electronic Control
Units (ECU), that meter fuel into the engine intake depending on
engine intake airflow. Typically, the volume of fuel is regulated
such that emissions are minimized and all of the fuel is completely
burned. The theoretical ratio of air to fuel for complete
combustion is 14.7 by weight for gasoline, called the
stoichiometric ratio. Theoretically, all available fuel combines
with all the intake air at the stoichiometric ratio. The unit
Lambda (.lamda.) is often used to represent the quotient of actual
air to fuel ratio over the region near the stoichiometric ratio.
Conventional electronic fueling systems typically include an oxygen
sensor in the exhaust that measures the oxygen concentration of the
exhaust. These oxygen sensors act as fuel cells that create an
output voltage by combining unburned hydrocarbons in the exhaust
with atmospheric oxygen. This results in a lambda/output transfer
curve where a .lamda. of 1.0 corresponds to an output voltage of
0.45V. Using the oxygen sensor, the fueling control system
regulates the fueling such that the resulting lambda is 1.0 at
medium load conditions using a feedback loop. The transfer curve of
a typical oxygen sensor is very steep where .lamda. is equal to
1.0, however, and significant variations in output voltage occurs
for slight variations in .lamda.. Accordingly, the measured voltage
cannot be used to measure other .lamda. values. At high load
conditions, a typical internal combustion engine produces maximum
power at lambda values <one (0.75 to 0.85). Conventional ECU
systems operate in an `open loop` mode under these conditions where
the volume of injected fuel is derived solely from pre-stored maps
that relate intake air mass to fuel mass without feedback. Because
engine aging and production variations change the actual air fuel
ratio of the engine, these pre-stored conditions are not always
correct for the particular engine. As a result, conventional
systems are limited in that severe inefficiencies can occur at high
load conditions. Many other wideband ions sensors experience
similar drawbacks.
[0004] Some recent developments in engine technology have resulted
in `lean-burn` systems that operate at lambda ratios greater than 1
(up to 1.1) to minimize fuel consumption and further minimize
emissions using special catalytic converters. Because ordinary
lambda sensors are not usable in these lambda regimens, a
`wide-band` or Universal Exhaust Gas Oxygen (UEGO) sensor has been
developed. UEGO sensors combine a small measuring chamber having an
orifice open to the exhaust stream, a standard oxygen sensor
(Nernst cell), and a pump cell. The pump cell is a solid-state
device of porous ceramic that allows oxygen to move between the
atmosphere and the measuring chamber. The direction and magnitude
of the current through the pump cell (often referred to as the pump
current) determines the direction and flow rate of oxygen ions. In
conventional systems, an active feedback loop is incorporated such
that the voltage at the oxygen sensor portion of the device is held
at the stoichiometric voltage. The pump current can then be used to
determine the .lamda. value over a wide range of ratios up to the
ratio for free air.
[0005] FIG. 1A is graphical illustration of a typical relationship
between the pump current and Lambda (.lamda.). As shown in FIG. 1A,
the resulting curve of pump current vs. lambda value (.lamda.) is
non-linear. Although the curve shape does not vary, manufacturing
tolerances in the sensors result in different magnitudes of pump
current vs. lambda (.lamda.) (i.e. the curve shifts). Attempts to
compensate for the variations include incorporating a calibration
resistor in the connector to the measuring cell sensor.
Unfortunately, this attempted solution does not address all of the
variations. Barometric air pressure and exhaust pressure also
influence the lambda/pump current relationship. Accordingly, the
outputs of theses sensors are not accurate. It is therefore
desirable to have a measurement method for oxygen sensors that is
self-calibrating and self-compensating for all the above
variations.
[0006] The pump current vs. lambda curve is also highly temperature
dependent. Typical UEGOs contain a heater element that maintains
the sensor at the desired operating temperature. The temperature
coefficient of the heater element is the quotient of change in
resistance (.DELTA.R) to the change in temperature (.DELTA.T).
Conventional techniques use the positive temperature coefficient of
the heater element to regulate input by operating the element at a
constant voltage. Because the temperature coefficient,
.DELTA.R/.DELTA.T, is fairly small at the operating temperature,
the resulting temperature regulation is not very precise. Depending
on the sensor, the pump cell impedance, the Nernst cell impedance,
or both have a much bigger temperature coefficient,
.DELTA.R/.DELTA.T, and would, therefore, allow more precise
temperature control. It would be more advantageous to control the
temperature of the pump cell. Unfortunately, at lambda values near
1, the pump current is very small or equal to zero and the pump
cell impedance can not be accurately measured on a low current. The
Nernst cell is typically physically bonded to the pump cell and,
therefore, the temperature of the Nernst cell and the pump cell
differ by a small amount. In order to measure the Nernst cell
impedance, a known fixed current or known fixed voltage have to be
impressed on the Nernst cell and the resulting voltage or current
then measured. Alternatively, a small alternating current (AC)
voltage or current can be impressed on the Nernst cell and the
resulting AC impedance measured. The first method requires stopping
the lambda measurement for a period of time and also requires
impressing the reverse charge on the Nernst cell to speed up
recovery. The second method does not interfere with the measurement
but requires low pass filters to remove the AC voltage or current
from the measured signal. The filters also remove the higher signal
frequencies which results in an inability to detect short transient
responses. Both methods measure the temperature of the Nernst cell,
not the pump cell. During operation, a temperature gradient between
the pump cell and the Nernst cell may occur and some temperature
control errors may result. Therefore there is a need for precise
pump cell temperature control while measuring lambda without
resorting to complicated circuitry to remove measurement
artifacts.
[0007] Further, conventional fuel metering techniques result in
significant pollution during the warm up period of the oxygen
sensor. In conventional systems where UEGO sensors are used, a
precise operating temperature must be attained before the UEGO
output value is reliable. This increases the time the fuel
injection system runs in `open loop` without knowledge of actual
air-fuel ratio. As a result, the time the engine creates
uncontrolled warm-up pollution is dependent on the sensor warm-up
time. Therefore, there also exists a need for an apparatus, system
and method for measuring an oxygen concentration which minimizes
the time before a reliable value is produced by the sensor.
[0008] Current wideband ion sensors such as wideband oxygen sensors
(WBO2 sensors) combine a Nernst cell reference sensor and a pump
cell in single package. A Nernst cell is an electrochemical cell
that produces a voltage that is nonlinearly proportional to the
difference in partial pressure of a measured gas between electrodes
of the cell. In a typical oxygen sensor application, the electrodes
are exposed to atmospheric air on an electrode on one side of a
measuring chamber and to an exhaust gas of an internal combustion
engine on the other electrode. A voltage is created by oxygen ions
migrating through the solid electrolyte material of the cell. The
pump cell is a Nernst cell where oxygen ion flow through the cell
is forced by an electrical current. If the current flows in one
direction, oxygen ions are transported from the outside air into
the sensor. If the current is reversed to the other direction,
oxygen ions are transported out of the sensor to the outside air.
The magnitude of the current determines the number of oxygen ions
that are transported each second.
[0009] The Nernst voltage is a voltage created as result of
electrochemical reaction in the cell. The cell acts basically as a
fuel cell. The Nernst voltage is created by the difference in
oxygen partial pressure between the two electrodes of the cell. The
Nernst equation describes it:
Voutput=(R*)(T)/(n)(F)*ln[(Po,air)/(Po,exh)]
[0010] where,
[0011] Voutput=O2 sensor's output voltage (0 to 1.0 volt is a
typical range)
[0012] R*=Universal Gas Constant=8.3143 [Joule/gram-mole*K]
[0013] T=Temperature of the exhaust gas [Deg K]
[0014] n=number of electrons involved in the reaction=4 in the NBO2
case
[0015] F=Faraday constant=96,480 [Coulomb/gram-mole]
[0016] Po, air=Partial pressure of O2 in the atmosphere
[Pascals]
[0017] Po, exh=Partial pressure of O2 in the exhaust gas at temp
[Pascals].
[0018] In conventional systems, both the Nernst cell and the pump
cell are mounted in a very small measuring chamber open with an
orifice (diffusion gap) to the exhaust gas. During a rich
condition, there is little or no oxygen and relatively high levels
of oxidizable combustion products within the measuring chamber. In
rich conditions, the WBO2 controller regulates the pump cell
current such that just enough oxygen ions are pumped into the
chamber to consume all oxidizable combustion products. This action
basically produces a stoichiometric condition in the measuring
chamber. In the stoichiometric condition, the Nernst reference cell
produces 0.45V. In a lean condition where there is excess oxygen,
the controller reverses the pump current so that all oxygen ions
are pumped out of the measuring chamber and a stoichiometric
condition returns. The pump cell is strong enough to pump all
oxygen out of the measuring chamber even if the chamber is filled
with free air.
[0019] The task of the WB controller in conventional systems,
therefore, is to regulate the pump current such that there is never
any oxygen nor oxidizable combustion products in the measuring
chamber. The required pump current is a measure of the Air/Fuel
ratio. Conventional wideband sensors, however, are difficult to
produce because multiple cells are combined in a small package.
Also, the small orifice to exhaust gas is susceptible to
contamination or blockage by exhaust particles limiting performance
of the sensor. In addition, conventional wideband sensors exhibit a
delay between Nernst reference cell output and changing pump cell
current because of the physical separation between the two devices.
Accordingly, an improved ion sensor is needed.
[0020] In addition, conventional oxides of Nitrogen (NOx) sensors
are implemented using with a Zirconium oxide (ZrO.sub.2) sensor.
Conventional ZrO.sub.2 sensors use Platinum (Pt) electrodes to
detect the O.sub.2 content of the gas to be measured. Pt electrodes
do not have the capacity to measure NOx, because Nitrous oxide
compounds are not disassociated by Platinum (Pt) alone. Alloys of
Rhodium and Platinum, however, can be used to disassociate the
nitrous oxide compounds. A sensor with an Yttrium stabilized
zirconium-oxide electrolyte will produce an output voltage that is
proportional to the difference in partial oxygen (O2) pressure
between the electrodes if the sensor is operating at the
appropriate temperature. When one electrode is exposed to air and
the other electrode exposed to exhaust gas, the output voltage
follows the Nernst relationship. When an electrical current is
passed through a cell formed in this way, the cell acts as oxygen
pump, where the oxygen current (in moles/second) is proportional to
the electrical current. Sensors where the exhaust side electrode is
constructed from a Pt--Rh alloy can also disassociate nitrous oxide
compounds. Accordingly, the Nernst Voltage can be represented
by:
Voutput=(R*)(T)/(n)(F)*ln[(Po,air)/((Po,exh)+(Pn,exh)]
[0021] where,
[0022] Voutput=O2 sensor's output voltage (0 to 1.0 volt is normal
range)
[0023] R*=Universal Gas Constant=8.3143 [Joule/gram-mole*K]
[0024] T=Temperature of the exhaust gas [Deg K]
[0025] n=number of electrons involved in the reaction=4 in the NBO2
case
[0026] F=Faraday constant=96,480 [Coulomb/gram-mole]
[0027] Po, air=Partial pressure of O.sub.2 in the atmosphere
[Pascals]
[0028] Po, exh=Partial pressure of O.sub.2 in the exhaust
[Pascals]
[0029] Pn, exh=Partial pressure of NOx compounds in the exhaust
[Pascals]
[0030] The NOx is decomposed at the Pt--Rh alloy electrode into N2
and O2 which causes a local increase in the O2 concentration at the
Pt--Rh electrode. The local increase is represented by Pn.
Accordingly, the partial pressure from NOx contributes to the
relationship. Conventional NOx sensors, however, are limited in
that when used in lean burn engines, such as diesel engines, the
leftover partial pressure of O.sub.2 in the exhaust is very high
compared to the partial pressure of NOx compounds. For example, the
leftover partial pressure of O.sub.2 in the exhaust is typically in
the single digit to multi digit percentage range while the partial
pressure of NOx compounds is in the parts per million (ppm) range.
Therefore, there is also a need for an ion sensor that extracts the
NOx content of the exhaust independently to the O.sub.2
content.
[0031] FIG. 1B is a block diagram of a conventional NOx sensor. The
gas to be measured (measured gas) 102 is received through a primary
diffusion gap 104 into a first measuring chamber 106. A first pump
cell 108 pumps oxygen ions from the first measuring chamber 106
either to atmospheric air or to the surrounding exhaust gas until
the remaining gas in the first measuring chamber 106 has a
relatively low oxygen concentration. A portion of this
oxygen-reduced gas diffuses through a secondary diffusion gap 110
into a secondary measuring chamber 112. A second pump cell 114 in
the secondary measuring chamber 112 includes an electrode 116
consisting of a Platinum (Pt) and Rhodium (Rh) alloy exposed to the
gas in the second measuring chamber 112. The Rhodium in this alloy
has catalytic properties that disassociate the Nitrous Oxides (NOx)
in the measurement gas within the second measuring chamber 112 into
Nitrogen (N.sub.2) and oxygen (O.sub.2). As a result, the oxygen
(O.sub.2) concentration in the second measuring chamber 112
increases slightly. A constant voltage is applied to the secondary
pump cell 114 and the current through that pump cell 114 is
measured. The NO.sub.2 content measurement is based on the current
through the secondary pump cell 114. An oxygen measuring cell 120
provides the feedback for the pump cell to regulate the pump
current in order to maintain a very low O2 concentration, while not
allowing the concentration to decrease to the point where the
voltage across the pump cell leads to electrolytic decomposition of
the ZrO2 solid electrolyte destroys the pump cell. This
conventional method, however, is limited in several ways. The
measured current is relatively small (within the nano-Ampere range)
and, as a result, is extremely susceptible to electromagnetic noise
contamination. Further, such conventional sensors are difficult to
manufacture due, at least partially, to multiple diffusion gaps.
Also, the gas concentration differences between the first measuring
chamber 106 and second measuring chamber 112 are very small.
Accordingly, the diffusion flow through the second diffusion gap
110 is significantly delayed resulting in a very slow response time
of the sensor.
[0032] Therefore, in addition to the needs described above for
wideband and NOx sensors, there is a need for a NOx sensor that is
easier to manufacture with increased performance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1A is graphical representation of a relationship
between pump current and an air to fuel ratio, Lambda (.lamda.),
for a typical Universal Exhaust Gas Oxygen (UEGO) sensor.
[0034] FIG. 1B is a block diagram of a conventional NOx sensor.
[0035] FIG. 2A is a block diagram of the oxygen monitoring
device.
[0036] FIG. 2B is a block diagram of an ion monitoring device where
the ion measuring device is a gas ion measuring device configured
to measure an oxygen ion concentration within a measured gas.
[0037] FIG. 3 is a schematic representation of the oxygen
monitoring device where the current managing unit is implemented
using an analog comparator circuit and an inverting amplifier
circuit.
[0038] FIG. 4 is a flow chart of a method of measuring an oxygen
concentration of a gas.
[0039] FIG. 5 is a flow chart of a method of varying the oxygen ion
flow within the measuring cell.
[0040] FIG. 6 is a flow chart of a method of calibrating an oxygen
measuring device.
[0041] FIG. 7 is a flow chart of a method of determining the oxygen
concentration of a gas by comparing the measured pulse width ratio
(PWM.sub.RATIO), to the pulse width ratio function.
[0042] FIG. 8 is a flow chart of a method of calibrating the heater
control unit.
[0043] FIG. 9 is a block diagram of a hand-held diagnostic device
suitable for embodying the oxygen measuring device.
[0044] FIG. 10 is a block diagram of a sensor system including a
sensor managing device connected to a wideband sensor where a
single electrochemical cell performs the functions of the sensor
cell and measuring cell.
[0045] FIG. 11 is a graphical representation of an exemplary pump
current and a corresponding cell voltage (V.sub.CELL).
[0046] FIG. 12 is a block diagram of a cross sectional view of
sensor including a single electrochemical cell and a diffusion gap
for use as the measuring opening.
[0047] FIG. 13 is a block diagram of a cross sectional view of a
sensor that includes a single electrochemical cell and a porous
membrane for the measuring opening.
[0048] FIG. 14 is a block diagram of an ion concentration sensor
including a measuring chamber, a primary electrochemical cell
system and a secondary electrochemical cell system.
[0049] FIG. 15 is a block diagram of an Oxides of Nitrogen (NOx)
sensor system including a NOx sensor having a pump cell, an oxygen
measuring cell, and a nitrogen sensitive electrochemical cell.
[0050] FIG. 16 is a block diagram of a cross section of a NOx
sensor for measuring ion concentrations in gases which is an
example of an implementation of the NOx sensor of FIG. 15
[0051] FIG. 17 is a schematic function diagram of a NOx measuring
system including the sensor managing device connected to the NOx
sensor of FIG. 16.
[0052] FIG. 18 is a graphical representation of an oxygen
concentration within the measuring chamber.
[0053] FIG. 19 is a graphical representation of a NOx pulse output
curve.
[0054] FIG. 20 is a block diagram of a sensor system including a
NOx sensor connected to a sensor managing device where the NOx
sensor includes a single oxygen electrochemical cell for performing
the functions of the oxygen sensor cell and the oxygen measuring
cell.
[0055] FIG. 21 is a block diagram of a cross section of a NOx
sensor for measuring ion concentrations in gases which is an
example of an implementation of the NOx sensor of FIG. 20.
[0056] FIG. 22 is a schematic function diagram of a NOx measuring
system including the sensor managing device 2004 connected to the
NOx sensor of FIG. 21.
[0057] FIG. 23 is flow chart of a method of managing a sensor
having a primary electrochemical cell system and a secondary
electrochemical cell system.
[0058] FIG. 24 is flow chart of a method of managing currents in
the sensor including a primary electrochemical cell system and a
secondary electrochemical cell system
[0059] FIG. 25 is a block diagram of a sensor system including a
sealed chamber sensor.
[0060] FIG. 26 is a block diagram of a cross section of an
electrochemical sensor including a sealed air chamber for measuring
exhaust gas.
DETAILED DESCRIPTION
[0061] As explained above, conventional sensor systems are limited
in several ways. These limitations are overcome in the exemplary
embodiment which provides an efficient, low cost, accurate method
for measuring an ion concentration of a fluid. An ion concentration
of a fluid is measured by varying a pump current through a
measuring cell based on an output of the measuring cell and
observing the pulse width ratio of the resulting square wave
representing the pump current. Further, in some circumstances, the
method described herein allows the sensor to be used earlier in the
warm-up period because the measurement method allows the
application of a correction factor that compensates for the fact
that the sensor has not yet achieved its desired operating
temperature. Also, the embodiments allow precise pump cell
temperature control while measuring lambda without resorting to
complicated circuitry to remove measurement artifacts. In addition
to determining oxygen ion concentrations, the embodiments can be
used to determine other gas ion concentrations. For example,
nitrogen sensors such as sensors that sense gaseous oxides of
nitrogen (NOx) can be connected to a current managing device and a
computer device to measure ion concentrations of gaseous oxides of
nitrogen such as NO and NO.sub.2 ion levels.
[0062] For some implementations, the functions of the pump cell and
the sensor cell are performed by a measuring cell including a
single electrochemical cell. A resistance voltage (V.sub.R)
resulting from the pump current and internal resistance of the cell
is subtracted from the total voltage across the cell to determine
the Nernst voltage of the cell. The Nernst voltage indicates an ion
concentration of the measured fluid. Where the measuring cell is
used to determine an oxygen concentration, thresholds for switching
the pump current through the measuring cell are derived from the
Nernst voltage. Where the measuring cell is used as part of a
primary electrochemical system of a NOx measuring system, the
Nernst voltage of the measuring cell is used as a reference to
evaluate an output from a nitrogen sensitive cell and determine
NOx.
[0063] In another NOX system embodiment, a pump cell and oxygen
measuring cell form a primary electrochemical system that pumps
oxygen into and out of a measuring chamber and provides a reference
voltage that is compared to the output signal of a nitrogen
sensitive electrochemical cell. A sensor managing device directs a
primary pump current through the pump cell at a first constant
primary pump current and a second constant primary pump current.
The sensor managing device uses the difference between the first
output signal from the measuring cell and the second output signal
of the nitrogen sensitive cell to direct a secondary pump current
through the nitrogen sensitive electrochemical cell at a first
constant secondary pump current and a second constant secondary
pump current. As described below, the nitrogen sensitive
electrochemical cell reduces NOx to nitrogen and oxygen to create a
local concentration of oxygen near the nitrogen sensitive
electrochemical cell. The duty cycles of the direction of the
secondary pump current or other related signals indicate the local
concentration of oxygen and, therefore, the concentration of
NOx.
[0064] In a sealed sensor embodiment, a measuring cell and
compensation cell are positioned adjacent to a sealed chamber. The
measuring cell and the compensation cell are electrically connected
in series with opposite polarity such that ions pumped into sealed
chamber by the measuring cell are pumped output of the sealed
chamber at, or nearly at, the same rate.
[0065] As discussed herein, oxides of nitrogen (NOx) include
compounds formed with nitrogen and oxygen. The compounds can be
reduced into oxygen ions and nitrogen ions. Accordingly, NOx are
examples of compounds formed from elements and that can be reduced
into ions of the elements. For combustion engines, NOx primarily
includes NO and NO.sub.2 although other compounds may be present in
some situations.
[0066] FIG. 2A is a block diagram of the ion monitoring device 200.
The ion monitoring device 200 may be implemented using any
combination of hardware, software and firmware. Various functions
and operations of the functional blocks described herein may be
implemented in any number of devices, circuits or elements. Any of
the functional blocks may be integrated in a single device and the
functions of the blocks may be distributed over several devices,
circuits and elements.
[0067] A measuring cell 202 includes at least a pump cell 204 and
an ion sensor cell 206 where a magnitude and direction of a pump
current 208 through the pump cell 204 is correlated to a flow of
ions 210 within the measuring cell 202. A measuring opening 212 of
the measuring cell 202 is positioned to receive a measured fluid
while a fluid opening 214 faces an ambient fluid. The measured
fluid and ambient fluid may be a gases or liquids. As discussed
below, for example, the measured fluid is a measured gas and the
ambient fluid is ambient air. The ion sensor cell 206 provides an
output signal based on the number of ions within the measuring cell
202. In response to the output signal, a current managing unit 216
varies the pump current between two constant current levels. A
first pump current is maintained by the current managing unit 216
until the output signal reaches a first threshold. When the first
threshold is reached, the current managing unit 216 directs the
pump current 208 in the opposite direction until the output signal
reaches a second threshold level. A computing device 218 monitors
the current fluctuation to determine an ion concentration of the
measured fluid. As discussed below, a suitable application of the
ion monitoring device 200 includes a gas ion monitoring device for
monitoring exhaust gas from a combustion engine to determine oxygen
concentrations for adjusting an air-fuel mixture. The ion
monitoring device, method, and system may be implemented as part of
any of several types of applications and systems and may be used to
measure any of numerous types of ions within a fluid medium. Some
examples include measuring ion concentrations of gaseous oxides of
nitrogen such as NO and NO.sub.2 ion levels, measuring carbon
dioxide levels, measuring gas ion concentrations in liquids such as
oxygen and carbon dioxide concentrations in water. Further, ion
concentrations of salts and elements such as lead within liquids or
gases may be measured in some situations. Accordingly, any of
numerous types of ion concentrations may be measured where the ion
sensor and current pump are responsive to the particular ions that
are measured. Further, as discussed below with reference to FIG.
20, FIG. 21 and FIG. 22, the measuring cell may be used as a
primary electrochemical system of a NOx measuring system where the
primary electrochemical system provides a first output signal that
used as a reference and compared to a second output signal of a
nitrogen sensitive electrochemical cell to determine NOx
concentrations.
[0068] After a calibration procedure is performed, the current
managing unit 216 varies the current 208 through the pump cell 204
between a constant positive current (Ip) and a constant negative
current (-Ip) based on the output signal of the ion measuring cell
206. When a negative current (-Ip) flows through the pump cell 204,
ambient fluid is received through the fluid opening 214 into the
measuring cell 202 through the pump circuit which results in an
increase of the ion concentration within the measuring cell 202. At
a high ion concentration of oxygen within the measuring cell 202,
the ion measuring cell 206 provides a low voltage signal output.
When an output signal lower threshold is reached, the current
managing unit 216, directs a positive current (Ip) through the pump
cell 204. When a positive current (Ip) flows through the pump cell
204, the ions in the measuring cell 202 flow out to the ambient
fluid. As the positive pump current 208 (Ip) continues to flow,
ions continue to flow out of the fluid opening 214. As a result,
the ion concentration continues to decrease. The output signal
continues to increase until an upper threshold is reached. In
response to detecting that the upper threshold has been reached,
the current managing unit 216 changes the direction of the pump
current 208. Examples of suitable values for the threshold include
values that maintain the ion measuring sensor 206 within a linear
range or substantially linear range. Also, the threshold values may
be same value in some circumstances to sample the concentration
with no hysteresis.
[0069] A square wave is formed between the positive and negative
current levels. The duration of the pump current 208 at positive
flow (Ip) and negative flow (-Ip) depends on the composition of the
measured fluid. Accordingly, the computing device 218 compares the
pulse width ratio (PWM.sub.RATIO) of the resulting square wave to a
known pulse width ratio function to determine the ion concentration
of the measured fluid.
[0070] FIG. 2B is a block diagram of an ion monitoring device 200
where the ion measuring device is a gas ion measuring device
configured to measure oxygen ion concentration within a measured
gas. The oxygen monitoring device 222 may be implemented using any
combination of hardware, software and firmware. Various functions
and operations of the functional blocks described herein may be
implemented in any number of devices, circuits or elements. Any of
the functional blocks may be integrated in a single device and the
functions of the blocks may be distributed over several devices,
circuits and elements.
[0071] In the oxygen monitoring device 222, the pump cell 204 and
the ion measuring cell 206 are responsive to oxygen ions. The ion
measuring cell 206 is an oxygen measuring cell 224. The measuring
cell 202 in the oxygen monitoring device 222, therefore, includes
at least the pump cell 204 and an oxygen sensor cell 224 where a
magnitude and direction of a pump current 208 through the pump cell
204 is correlated to a flow of oxygen ions 210 within the measuring
cell 202. The measuring opening 212 of the measuring cell 202 is
positioned to receive a measured gas while the fluid opening 214 is
an air opening 226 that faces ambient air. The oxygen sensor cell
224 provides an output signal based on the number of oxygen ions
within the measuring cell 202. In response to the output signal,
the current managing unit 216 varies the pump current between two
constant current levels. A first pump current is maintained by the
current managing unit 216 until the output signal reaches a first
threshold. When the first threshold is reached, the current
managing unit 216 directs the pump current 208 in the opposite
direction until the output signal reaches a second threshold level.
The computing device 218 monitors the current fluctuation to
determine an oxygen concentration of the measured gas. A suitable
application of the oxygen monitoring device 222 includes monitoring
exhaust gas from a combustion engine to determine oxygen
concentrations for adjusting an air-fuel mixture. The oxygen
monitoring device, method, and system may be implemented as part of
any of several types of applications and systems. As discussed
below, for example, the oxygen monitoring device 222 may be
implemented as a hand-held diagnostic device, as an original
equipment manufacturer (OEM) device within a vehicle, or as an
aftermarket device for permanent installation in a vehicle. In
addition to measuring oxygen, the oxygen measuring device and
method may be used to measure the oxygen concentration of exhaled
gases from a living being to determine the number of calories that
are being expended. Accordingly, the device and method discussed
with reference to FIG. 2B is only one example of the numerous
applications of the measuring system.
[0072] For the embodiment described with reference to FIG. 2B, the
oxygen sensor cell 224 is a Nernst cell (224) that is positioned
adjacent to a pump cell 204 in accordance with known techniques. It
is understood by those skilled in the art that although the
following description refers to a Nernst cell (224), the invention
may be implemented with other types of oxygen sensor cells 224
capable of providing an output signal based on the oxygen level in
a measured gas. After a calibration procedure is performed in
accordance with the procedure described below, the current managing
unit 216 varies the current 208 through the pump cell 204 between a
constant positive current (Ip) and a constant negative current
(-Ip) based on the output signal of the Nernst cell (224). When a
negative current (-Ip) flows through the pump cell 204, ambient air
is received through the air opening 226 into the measuring cell 202
through the pump circuit which results in an increase of the
concentration of oxygen within the measuring cell 202. At a high
concentration of oxygen within the measuring cell 202, the Nernst
cell (224) provides a low voltage signal output. When an output
signal lower threshold is reached, the current managing unit 216,
directs a positive current (Ip) through the pump cell 204. When a
positive current (Ip) flows through the pump cell 204, the oxygen
ions in the measuring cell 202 flow out to ambient air. Any
unburned carbons or fuel within the measuring cell 202 combine with
any remaining oxygen. As a result, the mixture of air and unburned
carbons within the measuring cell 202 decreases in oxygen
concentration and increases in fuel concentration. The output
signal increases through the transition point where no unburned
fuel and no excess oxygen are present in the measuring cell 202. At
this transition point, lambda is equal to 1.0 and the Nernst cell
(224) provides an output signal of approximately 450 mV. As the
positive pump current 208 (Ip) continues to flow, oxygen ions
continue to flow out of the air opening 214. As a result, the
concentration of oxygen continues to decrease and the concentration
of fuel increases in the measuring cell 202. The output signal
continues to increase until an upper threshold is reached. In
response to detecting that the upper threshold has been reached,
the current managing unit 216 changes the direction of the pump
current 208. For the embodiment of FIG. 2B, the upper threshold is
455 mV and the lower threshold is 445 mV. Other thresholds,
however, can be used where some suitable values include values
providing a range that includes the output signal for gas of
ambient air and which maintain the Nernst cell (224) within a
relatively linear potion of the lambda to voltage relationship. For
example, another suitable pair of values includes 440 mV and 460
mV. In some cases, the lower and upper thresholds may be the same
value.
[0073] As discussed above, a square wave is formed between the
positive and negative current levels. The duration of the pump
current 208 at positive flow (Ip) and negative flow (-Ip) depends
on the composition of the measured gas. Accordingly, the computing
device 218 compares the pulse width ratio (PWM.sub.RATIO) of the
resulting square wave to a known pulse width ratio function to
determine the oxygen concentration of the measured gas.
[0074] FIG. 3 is a schematic representation of the oxygen
monitoring device 222 where the current managing unit 216 is
implemented using an analog comparator circuit 304 and an inverting
amplifier circuit 306. The current managing device 216 may be
implemented using any combination and arrangement of hardware,
software and firmware. For the example of FIG. 3, the current
managing device 216 includes several hardware components including
resistors, operational amplifiers, analog switches, Zener diodes,
logic gates and other circuits. Those skilled in the art will
recognize the various substitutions that can be made for one or
more circuits or circuit elements by applying the teachings herein
in accordance with known techniques. Further, the operating values
may differ depending on the particular implementation of the
current managing device 216. Although the discussion with reference
to FIG. 3 is directed to oxygen sensors, the teachings can be
applied to other types of wideband sensors.
[0075] The inverting amplifier circuit 306 at least includes an
operational amplifier (U.sub.2) 308, an inverting input resistor
(R.sub.4) 310, and a non-inverting input resistor (R.sub.5) 312.
The voltage at the non-inverting input of the operational amplifier
(U.sub.2) 308 is maintained at voltage of U.sub.REF by a Zener
diode 314. U.sub.REF is equal to Vcc/2 which is approximately 2.5
volts for this example. The pump cell 204 in the measuring cell 202
is connected through an analog switch 316 between the output of the
operational amplifier (U.sub.2) 308 and the inverting input of the
operational amplifier (U.sub.2). The operational amplifier
(U.sub.2) 308, inverting input resistor (R.sub.4) 310 and the pump
cell 204 impedance (R.sub.pump) form the inverting amplifier 306
with a gain of -R.sub.pump/R.sub.4. The output of the operational
amplifier (U.sub.2) 308 is connected to the analog switch 316 that
connects the output of the operational amplifier 308 to the pump
cell 204 in response to the output level of an AND gate (U.sub.3)
318. Since the AND gate 318 provides an active "high" output when
the heater control unit 302 presents a "high" enable signal, the
analog switch 316 prevents current from flowing through measuring
cell 202 during warm up. Further, as explained below, during the
calibrate procedure, the analog switch 316 is opened during the
negative pump current 208 cycle resulting in a pump current 208
that alternates between a positive pump current (IP) and zero.
[0076] The inverting input of the operational amplifier (U.sub.2)
308 is connected to the output of the analog comparator circuit 304
through the inverting input resistor (R.sub.4) 310. The
non-inverting input resistor (R.sub.5) 312, a supply resistor
(R.sub.3) 320 and the Zener diode 314 form a voltage divider and
present a reference voltage of (Vcc/2+0.45V) to the inverting input
of an operational amplifier (U.sub.1) 322 of the analog comparator
circuit 304. For the example described with reference FIG. 3, the
reference voltage is 2.95 Volts since Vcc is 5 Volts. The positive
input of the operational amplifier 322 is connected to the output
of the Nernst cell (224) through a sensing resistor (R.sub.1) 324.
A feedback resistor (R.sub.2) 326 provides a voltage equal to
U.sub.REF+0.45V to the positive input of the operational amplifier
322. Therefore, the operational amplifier (U.sub.1) 322, the
resistor (R.sub.1) 324, and the feedback resistor (R.sub.2) 326
form the analog comparator circuit 304 operating with a hysteresis
voltage of approximately 10 mV.
[0077] The analog comparator circuit 304, the inverting amplifier
circuit 306 and the measuring cell 202 form an oscillator with a
variable pulse width modulation (PWM) ratio and a frequency that is
dependent on the response time of the measuring cell 202. The pump
current 208 alternates between +Vcc/(2*R4) and -Vcc/(2*R4). The
computing device 218 measures the times the output of U2 spends
above (t.sub.1) and below Vcc/2 (t2) and from that calculates the
PWM.sub.RATIO and .lamda. according to the function described
below. Lambda (.lamda.) is calculated at every transition of the
output of the comparator for the examples, herein. The Nernst cell
(224) provides an output signal approximately between 0.1 V and 0.7
V and the resulting (.lamda.) measurement frequency is about 7
octaves higher than the 3 dB point of the response frequency of the
oxygen sensor cell 206. Accordingly, the oxygen sensor cell 224
response frequency is well above the Nyquist frequency for the
examples discussed herein.
[0078] For the example of FIG. 3, the heater control unit 302
increases the temperature of the measuring cell 202 using a sensor
specific method and ramp-up schedule. After the measuring cell 202
has achieved its operating temperature, the "Ready" output of the
heater control unit 302 goes active providing a high ENABLE signal
to the AND gate (U.sub.3) which closes the analog switch 304. The
enable signal is also connected to an input of the computing device
218 and indicates to the computing device 218 that the measuring
cell 202 is ready for operation. The heater control unit 302 then
maintains a constant predetermined voltage over the heater element
or uses other (sensor specific) methods for temperature regulation.
For the example discussed with reference to FIG. 3, the pump cell
impedance is measured when the heater element 330 impedance is at
the minimum value. The pump cell impedance is maintained at the
measured value by continually monitoring the pump cell impedance
and adjusting the temperature with the heater element 330.
[0079] As described below with reference to FIG. 6, the computing
device 218 stores values in non-volatile memory corresponding to
the PWM ratio at the stoichiometric ratio (PWM.sub.ST) and the
pulse width ratio for air (PWM.sub.AIR). In the examples described
below with reference to FIGS. 4-8, a nominal lambda value having an
error on the order of +/-5% is calculated based on the calibration
values and the measured PWM.sub.RATIO. Because PWM.sub.ST is
dependent on the characteristics and age of the sensor much more
than on environmental conditions, the calibration process does not
need to be performed very often in most circumstances.
[0080] Based on these teachings, those skilled in the art will
recognize the various components, devices, and circuits elements
that can be used in the measuring device. An example of suitable
device that can be used for the operational amplifiers 308, 322
include the TLV2463 operational amplifier available from the Texas
Instruments company. Values for the inverting resistor (R.sub.4)
310 and the non-inverting (R.sub.5) resistor 312 are on the order
of a few hundred ohms. An example of suitable computing device 218
includes an 850 Family RISC 8-Bit Microcontroller. In some
circumstances, some or all of the functional blocks described above
may be implemented as an application specific integrated circuit
(ASIC). For example, heater control and current managing unit 216
and computing device 218 can be easily integrated into a mixed
signal ASIC with very few external parts.
[0081] FIG. 4 is a flow chart of a method of measuring an oxygen
concentration of a gas. The method may be performed with any
combination of hardware, software or firmware. For this example,
the method is performed in the oxygen measuring device 222.
Although the discussion with reference to FIG. 4 is directed to
oxygen sensors, the teachings can be applied to other types of
wideband sensors.
[0082] At step 402, a calibration procedure is performed. The
calibration procedure obtains the calibration values for
initializing the oxygen measuring device and may include values
related to the characteristics of the particular measuring cell 202
or related to environmental conditions. As explained below with
reference to the example of FIG. 6, values are obtained for
maintaining the pump cell 204 impedance, for establishing the pulse
width ratio function for calculating lambda, and for adjusting the
lambda value when the PWM ratio for a stoichiometric ratio
(PWM.sub.ST) is not zero. Other calibration values may include
parameters related to the frequency of a square wave of the pump
current 208 reflecting oxygen sensor characteristics.
[0083] At step 404, the oxygen ion flow is varied between a first
pump current and a second pump current based on the output signal
of the oxygen sensor cell 224. In this example, the ion flow is
varied by alternating the pump current 208 between a positive
constant current (IP+) and a negative constant current (IP-). The
analog switch 316 remains closed during the measurement
procedure.
[0084] At step 406, the pulse width ratio (PWM.sub.RATIO) of the
square wave formed by the pump current 208 is determined by the
computing device 218. In this example, the pulse widths (t.sub.1
and t.sub.2) of the square wave formed by the varying pump current
208 are measured using a crystal clock in the computing device 218.
Although individual values of a single pulse can be measured and
stored, the duration of the pulses resulting from the varying
current are averaged over a time period.
[0085] At step 408, the pulse width ratio (PWM.sub.RATIO) is
compared to the pulse width ratio function to determine the oxygen
concentration of the measured gas. In this example, the computing
device 218 applies the measured values to equations that utilize
the calibrated values.
[0086] FIG. 5 is a flow chart of a method of varying the oxygen ion
flow within the measuring cell 202. The flow chart of FIG. 5,
therefore, illustrates an exemplary method of performing step 404
of FIG. 4. Although the discussion with reference to FIG. 5 is
directed to oxygen sensors, the teachings can be applied to other
types of wideband sensors.
[0087] At step 502, the pump current 208 is directed in a positive
direction through the pump cell 204 at a constant magnitude. In the
oxygen monitoring device 200 described with reference to FIG. 3,
the analog switch 316 remains closed as positive voltage is applied
across the pump cell 204. The positive voltage is maintained until
the analog comparator circuit 304 triggers the inverting amplifier
308 to applying a negative voltage across the pump cell 204.
[0088] At step 504, the output signal from the oxygen sensor cell
224 is received. In the exemplary oxygen monitoring device 200, the
output of the oxygen sensor cell 224 is received through the
resistor (R.sub.1) 324 at the positive input of the operational
amplifier 322 of the analog comparator circuit 304.
[0089] At step 506, it is determined whether the output signal is
greater than or equal to the upper threshold. If the upper
threshold has not been reached, the method returns to step 502
where the constant positive pump current is directed through the
pump cell 204. If the upper threshold has been reached, the method
continues at step 508 where the current is reversed and a constant
pump current 208 is directed in the negative direction. As
discussed above with reference to example of FIG. 3, the current
managing device 216 includes an analog comparator circuit 304 and
an inverting amplifier circuit 306 to provide the constant current
until the thresholds are reached. The analog comparator circuit 304
triggers the reverse of the pump current 208 in response to the
detection that the thresholds have been reached. Therefore, the
positive pump current (IP+) is maintained until the output of the
oxygen sensor cell 224 reaches an upper threshold that causes the
output of the analog comparator circuit 304 to switch to a high
output changing the output of the inverting amplifier circuit
306.
[0090] At step 508, the pump current 208 is directed in a negative
direction. In response to the reversed voltage output of the
inverting amplifier circuit 306 the pump current 208 reverses
direction and becomes negative (-Ip).
[0091] At step 510, the current managing unit 216 receives the
output signal from the oxygen sensor cell 224. In the exemplary
oxygen monitoring device 222, the output of the oxygen sensor cell
224 is received through the resistor (R.sub.1) 324 at the positive
input of the operational amplifier 322 of the analog comparator
circuit 304.
[0092] At step 512, it is determined if the output signal is less
than or equal to the lower threshold. Of the lower threshold has
not yet been reached, the method returns to step 508 where the
current managing unit 218 continues to direct the pump current 208
in a negative direction through the pump cell 204. Otherwise, the
procedure returns to step 502, where the current is reversed to the
positive direction. Accordingly, in this example, the current
managing device 216 varies the current between 0.445 volts and 455
volts based on the output of the oxygen sensor cell 224. As the
pump current 208 is varied, characteristics of the resulting square
wave are measured and stored.
[0093] For the examples discussed herein, the computing device 218
monitors the time periods (t.sub.1 and t.sub.2) and if either of
the time periods exceeds an operating threshold, the computing
device 218 overwrites the ENABLE signal and disconnects the pump
cell 204 to prevent damage to the sensor. A diagnostic procedure is
performed to determine the fault condition.
[0094] FIG. 6 is a flow chart of a method of calibrating the oxygen
monitoring device 222. The method described with reference to FIG.
6 provides an example of a method of performing the calibration
step 402 of FIG. 4. The oxygen monitoring device 222 may be
calibrated in any number of ways and the particular calibration
method used may depend on a variety of factors such as the
characteristics of the particular sensor 202 and the data that will
be collected using the oxygen monitoring device 222. In this
example, the calibration procedure includes calibrating the heater
control unit 302 and determining the pulse widths of the varying
pump current 208 when the oxygen sensor cell 224 is exposed to free
air. Although the discussion with reference to FIG. 6 is directed
to oxygen sensors, the teachings can be applied to other types of
wideband sensors.
[0095] At step 602, the oxygen sensor cell 224 is exposed to free
air. In this example, the measuring cell 202 is placed in an area
where exposure to exhaust gases or other air borne impurities is
minimized. In some circumstances where the oxygen measuring device
222 is operating in a functioning vehicle, the computing unit
determines that the engine is in a coast down mode when the
resulting lambda value is above the lean burn limit for gasoline
and not changing over some period of time. When it is determined
that the vehicle is in a coast down mode, the computing device 218
performs the calibration procedure. If the computing device 218 is
the ECU itself, the coast down condition is already known and the
ECU, after the predetermined purge time of the exhaust system,
performs the calibration procedure for free air.
[0096] At step 604, it is determined whether the heater control
unit 302 should be calibrated. In this example, the heater control
unit 302 is calibrated during the powering up sequence. Examples of
other suitable situations that require the heater calibration
procedure to be performed include the replacement or reconnection
of the measuring cell 202 and the detection of certain measurement
errors. If heater calibration is required, the procedure continues
at step 606. Otherwise, the proceeds directly to step 608.
[0097] At step 606, the heater control unit 302 is calibrated. In
this example, a preferred heater impedance and a preferred pump
cell impedance corresponding to a preferred operating temperature
of the Nernst cell (224) are stored in memory. As discussed with
reference to FIG. 8, the Nernst cell impedance is maintained at a
target Nernst cell impedance for a suitable time period before the
preferred heater impedance and the preferred pump cell impedance
are measured and recorded.
[0098] At step 608, a sensor warm-up procedure is performed. In the
monitoring device described with reference to FIG. 3, the analog
switch 316 is initially opened during the sensor warm-up procedure.
In accordance with the appropriate heating timetable, power is
applied to the heater element 330 to increase the temperature. The
heater control unit 302 monitors the current and voltage across the
heating element 330 and determines the impedance of the heater
element 330. The heater impedance is compared to the preferred
heater impedance that was measured and stored during the heater
calibration procedure. When the heater control unit detects that
the heater impedance is equal to the preferred heater impedance,
the heater control unit 302 determines that the minimum operating
temperature of the oxygen sensor cell 206 has been reached. In
response to a determination that the desired operating temperature
is reached, the heater control unit 302 presents a "high" enable
signal at the "Ready" output. The AND gate (U3) 318 closes the
analog switch 316 when the ENABLE signal goes "high".
[0099] At step 610, the preferred operating temperature of the
Nernst cell is maintained. The preferred operating temperature is
maintained during the remainder of the oxygen sensor calibration
procedure as well as during operation of the oxygen monitoring
device 222. For the examples discussed herein, the pump cell 204
impedance R.sub.PUMP is constantly monitored during operation and
the heater control unit 302 is controlled to maintain a constant,
or nearly constant, preferred pump cell impedance. The preferred
pump cell impedance is retrieved from memory where it was stored
during the heater calibration procedure. An example of a suitable
method of controlling the heater control unit 302 includes using
pulse width modulation to increase or decrease the amount of power
dissipated by the heater element 330.
[0100] When the oxygen measuring device 222 is in an oscillating
mode and the current is varied, the voltage at the pump cell 204
(output of U.sub.2) is determined by Vcc, R.sub.PUMP, the resistor
R.sub.4 310, and the back-EMF of the pump cell 204. The output of
the operational amplifier (U.sub.1) 322 of the analog comparator
circuit 304 switches between 0V and Vcc. The heater control unit
302 samples the output of the operational amplifier (U.sub.2) 308
before and after each transition of the output of the operational
amplifier (U.sub.1) 322. The absolute value of the difference
between the voltage measured before and after each transition is
U.sub.DIFF. In some circumstances, the output of the operational
amplifier (U.sub.2) 308 is passed through a high pass filter (not
shown) of sufficiently high cut-off frequency. The filter output is
sampled immediately after the transition point and the absolute
value of resulting output voltage is equal to U.sub.DIFF.
[0101] The heater control unit 302 calculates the pump cell 204
impedance R.sub.PUMP in accordance with the following
relationship:
R.sub.PUMP=R.sub.4(U.sub.DIFF/Vcc) (1)
[0102] In some circumstances, the Nernst cell (224) impedance
(R.sub.N) is monitored as an alternative or in addition to
monitoring the pump cell 204 impedance. In order to monitor the
Nernst cell (224) impedance, the output voltage signal of the
Nernst cell (224) is passed through a high pass filter and
amplifier (not shown). The resulting filtered and amplified signal
is then sampled at the comparator transition point. The peak-peak
voltage, U.sub.NPP, is then calculated as the difference between
the sample voltage at low-high and high-low transition.
[0103] The voltage U.sub.NPP follows the equation:
U.sub.NPP=Vcc(R.sub.1+2R.sub.N)/R.sub.2 (2)
[0104] U.sub.NPP, therefore, linearly follows the Nernst cell (224)
impedance, R.sub.N, and is a convenient measurement for the Nernst
cell (224) impedance without the use of any filtering in the signal
path to influence the measured lambda signal. The resistors,
R.sub.1 and R.sub.2, are chosen such that the current through
R.sub.N is small enough to not influence the function of the Nernst
cell (224) and such that the U.sub.NPP at the Nernst operating
temperature and impedance is approximately 10 mV.
[0105] At step 612, the oxygen ion flow 210 is varied between a
positive current (Ip) and the negative current (-Ip) based on the
output signal of the oxygen sensor cell 224. An example of suitable
method of varying the current 208 is described above with reference
to FIG. 5.
[0106] At step 614, the pulse width ratio for air (PWM.sub.AIR) is
determined. For these examples, the pulse widths (t.sub.1AIR and
t.sub.2AIR) are determined for the positive current cycle and the
negative current cycle. The transition times of the square wave are
timed by a crystal clock within the computing device 218 to measure
the pulse widths. The values for the pulse widths are measure and
averaged over a sufficient time period such as one second, for
example, to calculate an average PWM.sub.AIR.
[0107] If the pulse width ratio for air is calculated during a
coast down condition, the computing device 218 determines when the
condition is reached before measuring the pulse widths of the pump
current 208. If the computing device 218 is an ECU in the system,
the ECU detects the condition based on parameters directly
available to the ECU such as throttle position and engine
speed.
[0108] At step 616, PWM.sub.AIR is stored in memory. Various
techniques may be used to store and retrieve calibration
information. For example, the pulse widths (t.sub.1AIR and
t.sub.2AIR) may be stored directly into memory and used for
calculating PWM.sub.AIR at a later time. Such a procedure may be
desired where the frequency of the square wave is used to further
compensate for pressure and temperature variations. By storing the
pulse width timing, frequency information is stored in addition to
the average pulse width ratio for air (PWM.sub.AIR).
[0109] At step 618, the oxygen ion flow 210 is varied between a
first current and second current based on the output signal of the
oxygen sensor cell 206. For the examples discussed herein, the
current 208 is varied between (IP) and zero. In a manner similar to
the method described above, the current 208 is varied from a first
current to a second current except that a zero current is used in
place of the negative current (IP-).
[0110] At step 620, the pulse width ratio for air when the second
current is zero (PWM'.sub.AIR) is determined. For the examples
discussed herein, the pulse widths (t'.sub.1AIR and t'.sub.2AIR)
are determined for the positive current cycle and the zero current
cycle. The transition times of the square wave are timed by a
crystal clock within the computing device 218 to measure the pulse
widths. The values for the pulse widths are measure and averaged
over a sufficient time period such as one second for example to
calculate an average PWM'.sub.AIR. To measure PWM.sub.AIR', the
computing device 218 sets the signal CALIBRATE high. The NAND-Gate
(U.sub.4) 328 together with AND-Gate (U.sub.3) 318 thus cause the
analog switch 316 to switch on only during the high phase of the
pump current 208. During the low phase, the analog switch 316 is
off and no pump current can flow.
[0111] At step 622, PWM'.sub.AIR is stored in memory. Various
techniques may be used to store and retrieve calibration
information. For example, the pulse widths (t'.sub.1AIR and
t'.sub.2AIR) may be stored directly into memory and used for
calculating PWM'AIR at a later time.
[0112] Other calibration procedures may be performed in some
situations. Calibration procedures for pressure and temperature
compensation, for example, may be performed by measuring and
storing frequency information corresponding to the pump current 208
at certain calibration conditions.
[0113] FIG. 7 is a flow chart of a method of determining the oxygen
concentration of a gas by comparing the measured pulse width ratio,
PWM.sub.RATIO, to the pulse width ratio function. The method
described with reference to FIG. 7 is an example of a method of
performing step 408 of FIG. 4. Although the discussion with
reference to FIG. 7 is directed to oxygen sensors, the teachings
can be applied to other types of wideband sensors.
[0114] At step 702, a preliminary oxygen concentration,
(.lamda..sub.PRE) is calculated. For the examples discussed herein,
the preliminary oxygen concentration (.lamda..sub.PRE) is
determined by the following equation:
.lamda..sub.PRE=P/(PWM.sub.AIR-PWM.sub.RATIO) (3)
where P=(1+PWM'.sub.AIR)(1-PWM.sub.AIR)/(1-PWM'.sub.AIR) (4)
[0115] The computing device 218 retrieves from memory the values
for PWM.sub.AIR, PWM.sub.RATIO, and PWM'.sub.AIR and applies the
above equations to calculate the preliminary oxygen concentration,
.lamda..sub.PRE. As explained below, P is equal to PWM.sub.AIR
where the pulse width ratio at the stoichiometric ratio
(PWM.sub.ST) is zero. Therefore, .lamda..sub.PRE is equal to
PWM.sub.AIR/(PWM.sub.AIR-PWM.sub.RATIO) where the PWM.sub.ST for
the particular sensor is zero.
[0116] At step 704, it is determined whether .lamda..sub.PRE is
less than one. If .lamda..sub.PRE is less than one, the procedure
continues at step 706. Otherwise, the procedure continues at step
708, where the oxygen concentration (.lamda.) of the gas is
determined to be equal to the preliminary oxygen concentration,
.lamda..sub.PRE.
[0117] At step 706, the oxygen concentration (.lamda.) of the gas
is determined to be equal to the sum of the preliminary oxygen
concentration (.lamda..sub.PRE) multiplied by a calibration factor
(M) and 1 minus the calibration factor
(.lamda.=(.lamda..sub.PRE)*M+(1-M)). For the examples discussed
herein, a calibration factor, M, for the brand and model of the
particular measuring cell 202 is derived through statistical
analysis of the measuring cell's 202 performance when exposed to a
gas with a known oxygen concentration. In some circumstances, a
calibration factor for each of several measuring cells is stored in
memory and applied to the particular model that is connected within
the oxygen measuring device 222. An example of typical value of M
is 0.71428.
[0118] FIG. 8 is flow chart of a method of calibrating the heater
control unit 302. The method discussed with reference to FIG. 8,
therefore, provides an example of a method for performing step 606
of FIG. 6. Although the discussion with reference to FIG. 8 is
directed to oxygen sensors, the teachings can be applied to other
types of wideband sensors.
[0119] At step 802, the heater element 330 impedance is monitored
as the temperature of the heater element 330 is increased. In the
monitoring device described with reference to FIG. 3, the analog
switch 316 is initially opened during the heater unit calibration
procedure. In accordance with the appropriate heating timetable,
power is applied to the heater element 330 to increase the
temperature. The heater control unit 302 monitors the current and
voltage across the heating element and determines the impedance of
the heater element. Based on stored information relating the heater
element impedance to the temperature of the heater element 330, the
heater control unit determines when the minimum operating
temperature of the oxygen sensor cell 224 is reached. In response
to a determination that the desired minimum operating temperature
is reached, the heater control unit 302 presents a "high" enable
signal at the "Ready" output. The AND gate (U3) 318 closes the
analog switch 316 when the ENABLE signal goes "high".
[0120] At step 804 it is determined whether the minimum operating
temperature has been reached. The procedure proceeds to step 806
when the minimum operating temperature is reached. Otherwise, the
heater temperature continues to be monitored at step 802 with the
analog switch 316 opened.
[0121] At step 806, the Nernst cell impedance is maintained at the
target Nernst cell impedance. The heater control unit 302 is
controlled such that the temperature is varied to maintain the
Nernst cell impedance at the target value. The target Nernst cell
impedance is a predetermined value that depends on the type and
brand of the measuring cell (sensor) 202 and is provided by the
sensor manufacturer. The Nernst cell impedance is held constant or
nearly constant for a minimum time to allow fluctuations in
temperatures and impedances to settle. An example of a suitable
settling time is ten seconds.
[0122] As described above, the Nernst cell (224) impedance is
monitored by passing the output voltage signal of the Nernst cell
(224) through a high pass filter and amplifier (not shown). The
resulting filtered and amplified signal is sampled at the
comparator transition point. The peak-peak voltage, U.sub.NPP, is
calculated as the difference between the sample voltage at low-high
and high-low transition in accordance with Equation 2.
[0123] At step 808, the preferred heater impedance and the
preferred pump cell impedance are measured and stored. For the
examples discussed herein, the pump cell impedance is calculated
based on Equation 1. As discussed above, the voltage at the pump
cell 204 (output of U.sub.2) is determined by Vcc, R.sub.PUMP, the
resistor R.sub.4, and the back-EMF of the pump cell 204 when the
oxygen measuring device 222 is in an oscillating mode. The output
of the operational amplifier (U.sub.1) 322 of the comparator 304
switches between 0V and Vcc. The heater control unit 302 samples
the output of the operational amplifier (U.sub.2) 308 before and
after each transition of the output of the operational amplifier
(U.sub.1) 322. The absolute value of the difference between the
voltage measured before and after each transition is U.sub.DIFF. In
some circumstances, the output of the operational amplifier
(U.sub.2) 322 is passed through a high pass filter (not shown) of
sufficiently high cut-off frequency. The filter output is sampled
immediately after the transition point and the absolute value of
resulting output voltage is equal to U.sub.DIFF.
[0124] Although various calibration factors and equations may be
used depending on the particular implementation of the oxygen
measuring device, the above equations are derived based on the
following analysis and assumptions for the examples discussed
herein. Those skilled in the art will recognize the modifications
based on the teachings herein.
[0125] The relationships between the various parameters are
described below with reference to equations 5-26 where the
following is assumed:
[0126] Q.sub.f is the required oxygen flow in and out of the
measuring cell 202 to maintain the Nernst cell (206) at the
transition point;
[0127] Q.sub.1 is an oxygen flow value out of the Nernst cell (224)
at the fixed constant current (Ip);
[0128] Q.sub.2 is an oxygen flow value into the Nernst cell (224)
at the fixed constant current (-Ip);
[0129] t.sub.1 is the oxygen pump time (Q.sub.1 flow) required to
switch the Nernst cell (224) from 0.445V to 0.455V; and
[0130] t.sub.2 is the oxygen pump time (Q.sub.2 flow) required to
switch the Nernst cell (224) from 0.455V to 0.445V.
[0131] For the forgoing assumptions, therefore, the Nernst cell
(206) voltage is 0.45V with an alternating current (AC) component
of 10 mVpp. The resulting Q.sub.f is:
Q.sub.f=(Q.sub.1*t.sub.1-Q.sub.2*t.sub.2)/(t.sub.1+t.sub.2) (5)
[0132] The timing relationships can be expressed as
PWM.sub.RATIO=(t.sub.1-t.sub.2)/(t.sub.1+t.sub.2) (6)
[0133] Using 1 and 2, equation 1 can be rewritten as:
Q.sub.f=[(Q.sub.1+Q.sub.2)*PWM.sub.RATIO+Q.sub.1-Q.sub.2)]/2
(7)
[0134] Pump flow ratio (Q.sub.RAT) can be expressed as:
Q.sub.RAT=(Q.sub.1-Q.sub.2)/(Q.sub.1+Q.sub.2) (8)
[0135] At changing air pressure, Q.sub.1 and Q.sub.2 change
approximately proportionally and, therefore, Q.sub.RAT stays nearly
constant. The same holds true for temperature changes. Accordingly,
Q.sub.RAT is independent of temperature.
[0136] In some circumstances, Q.sub.RAT may change when the sensor
ages and, therefore, the sensor may need to be periodically
calibrated to maintain optimal performance.
[0137] If Q.sub.1 and Q.sub.2 are known and are constants, the
oxygen flow rate and Lambda, (.lamda.) is determined from the
timing relationship, PWM.sub.RATIO, which is measured. Q.sub.1 and
Q.sub.2 are constant if the pump current 208, temperature, exhaust
pressure, barometric pressure and oxygen concentration in air are
constant. For the examples discussed herein, the pump current 208
and temperature are held constant through careful circuit design.
For the analysis described herein, the atmospheric oxygen
concentration is assumed to be constant at 20.9%. Barometric
pressure effects are compensated through calibration. The effect of
exhaust pressure tends to modify both, Q.sub.1 and Q2 by an equal
factor and also modifies the response time of the oxygen sensor
cell 206 because more or less oxygen ions are present at the oxygen
sensor cell 206 surface depending on pressure.
[0138] As described above, the oxygen monitoring device 222
measures oxygen flow by switching the pump current 208 between a
constant positive and negative value. The absolute value for this
constant pump current value is chosen such that it is greater than
the absolute value of the pump current 208 required for free
air.
[0139] The above equation is linear and can be determined with two
known points. The time values t.sub.1 and t.sub.2 are measured by a
crystal controlled microprocessor or timer circuit which allows the
accurate determination of Lambda, (.lamda.), once the two
calibration points are known.
[0140] A stoichiometric exhaust mixture does not require any
corrective oxygen flow and the steady state pump current 208 is,
therefore, equal to zero. This condition is used to determine one
of the calibration points, the stoichiometric pulse width ratio,
PWM.sub.ST.
[0141] As described above, a second calibration point is obtained
by measuring the pulse width ratio when the measured gas is air.
The measuring cell 202 is exposed to free air. If the measuring
cell 202 is not installed in a vehicle, the measuring cell is
placed in an area exposed to free air. If the measuring cell 202 is
installed in a vehicle, the calibration for free air is performed
when the vehicle has not been in operation for an adequate time and
all the exhaust gases have dissipated or when the vehicle is in a
cost-down mode. During the coast-down mode, the throttle on the
engine is completely closed and engine speed is above a
predetermined value. In this case, a typical ECU will not inject
any fuel because no power output is required from the engine and
further fuel can be saved. The pump cell 204 is then driven with a
total flow value Q.sub.F that is high enough to pump all oxygen
from the air in the measuring chamber.
[0142] From equations 5 through 8 follows:
PWM.sub.ST=-Q.sub.RAT. (9)
[0143] The lambda value, .lamda., calculated from exhaust oxygen
concentration can be expressed as:
.lamda.=Air Oxygen content/(Air Oxygen content-Excess Oxygen)
(10)
Note that the value Excess Oxygen in Equation 6 can have negative
values if all oxygen is consumed but unburned or partially burned
fuel is still present.
[0144] To examine the oxygen flow rate instead of volume, t is
eliminated by division:
.lamda.=Q.sub.f(AIR)/(Q.sub.f(AIR)-Q.sub.f): (11)
[0145] applying equations 7, 8, 9, and 11:
.lamda.=(PWM.sub.AIR-PWM.sub.ST)/(PWM.sub.AIR-PWM.sub.RATIO)
(12)
[0146] As described above, a second free air PWM ratio
(PWM'.sub.AIR) is measured by switching the pump cell 204 between
Q.sub.1 and no current (Q.sub.2=0) during free air calibration.
[0147] PWM.sub.ST is calculated during calibration from PWM.sub.AIR
and PWM'.sub.AIR according to the following formulas:
[0148] From equation 7,
2*Q.sub.f=(Q.sub.1+Q.sub.2)*PWM.sub.AIR+Q.sub.1-Q.sub.2 (13)
2*Q.sub.f=Q.sub.1*PWM'.sub.AIR+Q.sub.1 (14)
[0149] Where PWM'.sub.AIR is measured when switching between
Q.sub.1 and no current instead of Q.sub.1 and Q.sub.2.
P=PWM.sub.AIR-PWM.sub.ST. (15)
[0150] From equations 13 and 14:
P=(1+PWM'.sub.AIR)*(1-PWM.sub.AIR)/(1-PWM'.sub.AIR) (16)
PWM.sub.ST=PWM.sub.AIR-P (17)
[0151] Applying equation (12):
.lamda.=P/(PWM.sub.AIR-PWM.sub.RATIO) (18)
[0152] As explained above, PWM.sub.AIR is measured by exposing the
sensor to free air at the appropriate operating temperature and, in
some circumstances, frequency information is used for determining
compensation factors. The following analysis demonstrates the
relationship between frequency and other parameters.
[0153] Returning to equation 8, if Q.sub.1=Q.sub.2, Q.sub.RAT(and
therefore PWM.sub.ST) becomes zero. The actual sampling frequency
is dependent on the full flow ratio, Q.sub.F.
[0154] Equation 8 then changes to:
Q.sub.f=Q.sub.F*PWM.sub.RATIO. (19)
[0155] Equation 12 becomes
.lamda.=PWM.sub.AIR/(PWM.sub.AIR-PWM.sub.RATIO) (20)
[0156] Q.sub.F is a function of the pump current 208, Ip, and,
therefore, QF=f (Ip). If Q.sub.F for a constant Ip changes because
of exhaust pressure changes, the measured PWM.sub.RATIO becomes
PWM'.sub.RATIO for the same corrective flow, Q.sub.f.
[0157] With exhaust gas pressure or temperature changes Q.sub.1 and
Q.sub.2 change by a factor K in a first approximation.
[0158] Equation 8 then becomes:
Q.sub.f=K[(Q.sub.1+Q.sub.2)*PWM'.sub.AIR+Q.sub.1-Q.sub.2)]/2
(21)
where
Q.sub.1*t.sub.1=K*Q.sub.1*t.sub.1' (22)
Q.sub.2*t.sub.2=K*Q.sub.2*t.sub.2' (23)
[0159] The measurement frequency f is determined by:
f=1/(t.sub.1+t.sub.2) (24)
f'=1/(t.sub.1'+t.sub.2') (25)
[0160] From equations 20, 21, 22 and 23 follows:
K=f'/f (26)
[0161] Because f is constant when all other environmental
conditions are constant, this calculation can be used to correct
for temperature and/or pressure changes. Equation 8 then
becomes:
.lamda.=(PWM.sub.AIR-PWM.sub.ST)/(PWM.sub.AIR-(1-K)*PWM.sub.ST-K*PWM'.su-
b.RATIO) (27)
[0162] and equation 18 becomes:
.lamda.=PWM.sub.AIR/(PWM.sub.AIR-K*PWM'.sub.RATIO) (28)
[0163] These equations, therefore, allow the application of a
pressure compensation factor, K to compensate for pressure or
temperature changes. Under extreme circumstances, Q.sub.1 and
Q.sub.2 do not change equally by the same factor K. In some
situations, therefore, the normalized frequency deviation f''/f is
used as an index into an experimentally derived lookup table to
extract the accurate deviation factor K':
K'=func(f'/f). (29)
[0164] The calculated Lambda value can thus be corrected for
exhaust pressure changes without the use of separate sensors to
measure exhaust pressure once a normalized frequency/lambda table
is experimentally determined for a given sensor type.
[0165] Conventional commercially available packaged measuring cells
202 often have temperature dependent parasitic resistances to the
virtual ground of the pump cell 204 and Nernst cell (224). This
parasitic resistance must be addressed through software or
circuitry in order to apply pressure compensation methods described
above with many commercially available measuring cells 202.
[0166] The forgoing equations and analysis may be applied to other
implementations of the invention in ways other than described above
and the teachings described herein may be applied to a variety of
formats, implementations and configurations. As explained above,
the hardware and software may be modified to accommodate a variety
of factors. For example, the analog switch 316 can be eliminated
where the operational amplifier (U.sub.2) 308 provides a tri-state
output. Also, the analog switch 316 can be connected within the
oxygen measuring device 222 before the inverting resistor (R.sub.4)
310 instead of connecting to the output of the operational
amplifier (U.sub.2) 308. The operational amplifier (U.sub.2) 308
may also provide a tri-state output. In addition, the heater
controlling unit 302 may be integrated as part of the computing
device 218.
[0167] Further, the Zener diode 314 may be replaced with a digital
to analog (D/A) converter or a potentiometer in some circumstances.
The references voltage U.sub.REF could thereby be set such that the
pulse width ratio at the stoichiometric ratio, PWM.sub.ST is
exactly zero. In such a circumstance, the equation used to
calculate A is:
.lamda.=PWM.sub.AIR/(PWM.sub.AIR-K*PWM') (30)
[0168] In some circumstances, frequency information is analyzed to
provide other useful information or data in accordance with the
analysis above. For example, because the response time of a
measuring cell 202 changes with aging, the oscillating frequency is
used directly as a measurement to determine the need for
replacement. When a lower threshold frequency is reached, the
computing device 218 may provide a warning that the sensor should
be replaced. The frequency analysis is preferably performed when
the free-air value is recalibrated because the environmental
conditions are comparable (f' and f in equation 27 are equal) and
the frequency change is due to aging of the sensor.
[0169] FIG. 9 is a block diagram of a hand-held diagnostic device
suitable for embodying the oxygen measuring device 222. As
mentioned above, the oxygen measuring device 222 may be implemented
as any of several configuration and devices. The oxygen measuring
device 222, for example, may be integrated as an OEM device in a
vehicle fuel system. Further, the oxygen measuring device 222 may
be part of an in-vehicle aftermarket fueling system or diagnostic
system. Other devices and uses will be readily apparent to those
skilled in the art based on the teachings herein.
[0170] The hand-held diagnostic device 900 includes a housing 902,
a display 904, connectors 906-912, and buttons (or other type of
switches) 912, 914 that provide interfaces to the computing device
218 and the current managing device 216. The display allows the
user to view information regarding the status to the hand-held
diagnostic device 900. In the hand held device 900, the connectors
906-912 include a serial port 912 for connecting to an external
computer, analog output connector 908 for supplying an analog
signal corresponding to the measured A, an auxiliary sensor
interface 919, and a sensor connector 906. Other connectors such as
a power connector for receiving DC supply power, for example, are
also included in some circumstances. A calibrate button 908
connected to the computing device 218 provides a user interface for
initiating the calibration procedure. A record button 914 provides
a user interface for initiating a record procedure that allows
several seconds of data to be stored in memory. An example of
another button or switch that may be used includes an on-off switch
(not shown). The buttons and connectors are connected to the
computing device 218 and other circuitry and provide interfaces
between the user, the measuring device 222, the measuring cell 202
and other external equipment.
[0171] Therefore, the system, apparatus and method for measuring
the oxygen concentration of gas provides a cost effective,
efficient and accurate way to monitor a gas having several
advantages over conventional systems. The techniques described
herein provide a simplified design since no analog to digital (A/D)
conversion is required for an oxygen concentration (.lamda.)
measurement. Further, no calibration resistor is required in the
measuring cell sensor to compensate for sensor tolerances which
results in simplified production and lower production costs. Wide
tolerances of the measuring cell 202 itself are acceptable,
resulting in higher possible production yield. Because no precision
resistors or other precision parts are required, circuit cost is
minimized. The oxygen monitoring device 222 self-compensates for
pressure and temperature variations. The measurement process is
converted to the time-domain, instead of an analog current/voltage
domain. By using standard crystal time bases, as is typical in
digital designs, temperature and age-related drifts are eliminated
because crystal time bases have tolerances of <10.sup.-6
compared to <10.sup.-2 for typical resistors. Measurement
results are linear to 1/Lambda and independent of the lp/Lambda
curve of the sensor. Calibration is convenient and uses only air as
a reference gas.
[0172] Although the discussion with reference to FIG. 9 is directed
to a hand-held diagnostic device suitable for embodying the oxygen
measuring device 222, the teachings can be applied to implement a
handheld device for use with other types of wideband sensors. For
example, the device 900 may be configured to connect to measuring
cell that is responsive to nitrogen or to gaseous oxides of
nitrogen.
[0173] FIG. 10 is a block diagram of a sensor system 1000 including
a sensor managing device 1004 connected to a wideband sensor 1002
in accordance with an example where a single electrochemical cell
1006 performs the functions of the sensor cell and the pump cell.
The ion concentration measuring system 1000 includes a sensor that
has a measuring cell (formed by a single electrochemical cell) and
a sensor managing device that varies a pump current through the
electrochemical cell between a first constant current and a second
constant current in accordance with a cell voltage at the
electrochemical cell. A measured fluid is received through a
measuring opening 1010 into a measuring chamber 1008 of the sensor.
The electrochemical cell 1006 moves ions between the measuring
chamber 1008 and an ambient opening 1012 exposed to an ambient
fluid, such as air, based on a pump current flowing through the
electrochemical cell 1006. The sensor measuring device 1004
determines the ion concentration of the measured fluid based on a
cell voltage (V.sub.CELL) 1018 at the electrochemical cell 1006.
The internal resistance of the electrochemical cell 1006 is
determined and subtracted from the cell voltage 1018 to obtain the
Nernst voltage of the electrochemical cell 1006 which indicates the
ion concentration of the measured fluid.
[0174] The sensor system 1000 may be implemented using any
combination of hardware, software and firmware. Various functions
and operations of the functional blocks described herein may be
implemented in any number of devices, circuits or elements. Any of
the functional blocks may be integrated in a single device and the
functions of the blocks may be distributed over several devices,
circuits and elements.
[0175] The sensor 1002 includes an electrochemical cell 1006
connected within a measuring chamber 1008 having a measuring
opening 1010 and an ambient opening 1012. A magnitude and direction
of a pump current 1014 through the electrochemical cell 1006
dictates a flow of ions 1016 within the electrochemical cell 1006.
The measuring opening 1010 of the measuring chamber 1008 is
positioned to receive a measured fluid while an ambient opening
1012 faces an ambient fluid. The measured fluid and ambient fluid
may be a gases or liquids. For the example presented herein, the
measured fluid is a measured gas and the ambient fluid is ambient
air where the measured gas is oxygen. The electrochemical cell 1006
is any device, component, or element that changes the ion
concentrations within the measuring chamber 1008 based on the pump
current 1014 flowing through the electrochemical cell 1006 and
presents a voltage (V.sub.CELL) 1018 that is correlated to the ion
concentration. The electrochemical cell 1006 is similar to a pump
cell in conventional sensors. For this example, the electrochemical
cell 1006 is a Nernst cell responsive to oxygen ions. The
electrochemical cell 1006, however, may be responsive to other
gases in some circumstances such as gaseous oxides of nitrogen
(NOx), for example.
[0176] When a constant electrical current is forced through the
electrochemical cell 1006, a voltage is created at the cell which
is the sum of the Nernst voltage and the voltage drop (resistance
voltage) created by internal resistance of the electrochemical cell
1006. The internal resistance is the real impedance of the cell
sometimes referred to as the Ohmish impedance. The resistance
voltage (V.sub.R) results from the pump current flowing through the
internal resistance. A Nernst voltage indicates the oxygen
concentration in the measuring chamber and is equal to the
difference between the total electrochemical cell voltage
(V.sub.CELL) and the resistance voltage (V.sub.R). The Nernst
voltage, therefore, can be calculated by subtracting the resistance
voltage (V.sub.R) from the electrochemical voltage (V.sub.CELL).
For this example, the sensor managing device 1004 continually
switches the pump current 1014 between positive and negative
constant currents, measures the cell voltage, and determines the
oxygen concentration based on the Nernst voltage by subtracting the
resistance voltage (V.sub.R). The sensor managing device 1004
includes a current managing unit 216 and a computing device 218
where the current managing unit controls 216 the current flow and
measures the cell voltage. Accordingly, operation in the example
discussed with reference to FIG. 10 is similar to the operation of
the example described with reference to FIG. 2A, FIG. 2B, and FIG.
3 except that the pump cell and the measuring cell are replaced
with a single electrochemical cell. The pump cell in the example of
FIG. 10, therefore, also acts as the measuring cell.
[0177] The electrochemical cell 1006 provides an output signal
based on the number of ions within the measuring chamber 1008. In
response to the output signal, the sensor managing device 1004
varies the pump current between two constant current levels. A
first pump current is maintained by the current managing unit until
the output signal reaches a first threshold. When the first
threshold is reached, the current managing unit 1004 directs the
pump current 1014 in the opposite direction until the output signal
reaches a second threshold level. A computing device monitors the
current fluctuation to determine an ion concentration of the
measured fluid (gas). The pulse width ratio (duty cycle) of the
resulting oscillation is used as an indicator of oxygen flow
through the pump cell. Evaluations of the pulse width ratio of the
current signal and other related waveforms and signals may be used
to determine the oxygen concentration. As discussed above, a
suitable application of the sensor system 1000 includes a gas ion
monitoring device for monitoring exhaust gas from a combustion
engine to determine oxygen concentrations for adjusting an air-fuel
mixture. The ion monitoring device, method, and system may be
implemented as part of any of several types of applications and
systems and may be used to measure any of numerous types of ions
within a fluid medium. Some examples include measuring ion
concentrations of gaseous oxides of nitrogen such as NO and
NO.sub.2 ion levels, measuring carbon dioxide levels, measuring gas
ion concentrations in liquids such as oxygen and carbon dioxide
concentrations in water. Further, ion concentrations of salts and
elements such as lead within liquids or gases may be measured in
some situations. Accordingly, any of numerous types of ion
concentrations may be measured where the ion sensor and current
pump are responsive to the particular ions that are measured.
Further, as discussed below with reference to FIG. 20, FIG. 21 and
FIG. 22, the measuring cell may be used as a primary
electrochemical system of a NOx measuring system where the primary
electrochemical system provides a first output signal that used as
a reference and compared to a second output signal of a nitrogen
sensitive electrochemical cell to determine NOx concentrations.
[0178] An example of suitable sensor managing device 1004 includes
an apparatus that is configured to connect to the measuring cell
(electrochemical cell 1006) through an interface 1022 and includes
circuitry forming a current managing unit 1018 and a computing
device 1020. The interface 1022 may include an electrical
connector, direct cable connection, or other electrical contact
arrangement for conveying signals between the sensor 1002 and the
sensor managing device 1004. The current managing unit 1018 is
configured to receive the output signal 1018 based on the ion
concentration within the measured fluid within the measuring
chamber. The current managing unit 1018 is further configured to
adjust an ion flow between the electrochemical cell 1006 and
ambient fluid by varying, in accordance with the output signal
1018, the pump current 1014 flowing through the pump cell of the
electrochemical cell 1006 between a first constant current and a
second constant current. The computing device 1020 is configured to
determine the ion concentration of the measured fluid in accordance
with the pulse width ratio of a square wave of the pump current.
Accordingly, depending on the particular implementation, the
computing device 1020 may evaluate any of numerous signals or
waveforms related to or derived from the pump current 1014.
[0179] Therefore, the measured voltage is used to trigger the
reversal of pump current. For example, if during a positive pump
current (I.sub.P) the Nernst voltage
(|V.sub.CELL|-|R*I.sub.P|)>=0.5 Volt, then the pump current
(I.sub.P) is reversed, R (internal resistance) is calculated as
described below, and the process continues with a negative constant
pump current until the Nernst Voltage is <=0.4V. Then the pump
current (I.sub.P) is polarized back to positive and so on. In this
example, the Hysteresis voltage is 0.1V (0.5V-0.4V). Different
Hysteresis Voltages can be used. In some cases, the same value may
be sued for both thresholds.
[0180] For this example, the internal resistance (R) of the
electrochemical cell is determined by measuring the voltage change
at the cell at the transition point between positive and negative
current and/or between negative and positive current through the
cell. Since the pump current is switched between constant positive
and negative currents, the resistance is calculated based on Ohms
law.
[0181] The internal resistance is dependent on the temperature of
the electrochemical cell. At a polarity reversal of the pump
current, the cell has not had time to react and has not pumped any
oxygen in the new direction. So the oxygen concentration
difference, which determines the Nernst voltage, has not yet
changed by a significant amount. Accordingly, the voltage change at
the cell is at least mostly caused by the change in current. Based
on the difference in current and difference in voltage, the
internal resistance is determined based on the relationship
R.sub.CELL=.DELTA.V.sub.CELL/.DELTA.I.sub.P, where
.DELTA.V.sub.CELL is the difference in voltage at the cell and
.DELTA.I.sub.P is the difference in pump current. The internal
resistance R.sub.CELL is used to determine the voltage drop
(V.sub.R) due to the internal resistance R.sub.CELL based on the
Ohms Law, V.sub.R=R.sub.CELL*I.sub.P. The resistance voltage
(V.sub.R) is subtracted from the actual voltage (V.sub.CELL) at the
electrochemical cell for the remainder of the cycle in subsequent
calculations to determine the Nernst voltage and consequently the
ion concentration. In most applications, the voltage change
.DELTA.V.sub.CELL can be measured up to a few microseconds before
and after the current transition. The difference between the
voltage immediately before the polarity reversal and immediately
after is the .DELTA.V.sub.CELL voltage change. A suitable technique
for measuring the .DELTA.V.sub.CELL voltage includes using a sample
and hold circuit.
[0182] The above discussion can be applied to an example as
follows. If the absolute value of the pump current is 5 mA, and
measured .DELTA.V.sub.CELL is 0.8 Volts (Ip jumps from +5 mA to -5
mA. As delta Ip is 10 mA (+5 mA-(-5 mA)), R.sub.CELL is 80 Ohms. If
the measuring chamber is slightly richer than stoichiometric
(Nernst voltage is 0.5V), then the actual measured cell voltage at
a jump from +5 mA to -5 mA jumps from 0.9V to 0.1 V. Now the pump
current is negative (-5 mA) and the pump cell pumps oxygen ions
into the measuring chamber(s). This makes that chamber gradually
leaner and the pump cell voltage decays. At the lower threshold of
0V (with a 0.1V hysteresis) the polarity of the current is reversed
again and the voltage on the cell jumps to 0.8V (0.4V
Nernst+V.sub.R of 0.4V). Now oxygen ions are pumped again out of
the chamber and the voltage across the pump cell rises again until
it reaches the upper threshold of 0.9V (0.5V Nernst+V.sub.R of
0.4V) and so on.
[0183] FIG. 11 is a graphical representation of an example of a
pump current 114 and a corresponding cell voltage (V.sub.CELL)
1018. As the pump current 1014 is switched between a constant
positive current 1102 and a constant negative current 1004, the
voltage (V.sub.CELL) across the electrochemical cell (pump cell)
also oscillates between negative and positive voltage. As discussed
above, a portion (V.sub.R) 1106 of the total voltage (V.sub.CELL)
is due to the internal resistance (R) and is referred to as the
resistance voltage (V.sub.R) 1106. After the pump current is
reversed, the opposite resistance voltage appears at the cell and
begins to approach a threshold at the total cell voltage. The sum
of the resistance voltage and the Nernst voltage 1108 is equal to
the total cell voltage (V.sub.CELL).
[0184] FIG. 12 is a block diagram of a cross sectional view of a
sensor 1200 where the sensor 1200 includes a single electrochemical
cell and a diffusion gap 1202 for use as the measuring opening
1010. A measuring side electrode is exposed to the measuring
chamber 1008 and an atmospheric electrode is exposed to the
atmospheric fluid 1210 such as air. As explained above,
conventional wideband sensors exhibit a delay between Nernst
reference cell output and changing pump cell current because of the
physical separation between the two components. The delay is not
applicable in the example of FIG. 12 because the active electrode
surface of the pump cell also reacts directly without delay on the
measured gas. This further increases the measurement speed.
Accordingly, the embodiment illustrated in FIG. 12 is an example of
a wideband sensor constructed without a separate sensor reference
cell but otherwise in accordance with conventional techniques.
[0185] FIG. 13 is a block diagram of a cross sectional view of a
sensor 1300 where the sensor 1300 includes a single electrochemical
cell and a porous membrane 1302 for the measuring opening 1010. In
the implementation shown in FIG. 13, measuring chamber and
diffusion gap are omitted. The electrode of the pump cell that is
exposed to exhaust gas is covered with an inert porous material
that allows weak diffusion of exhaust gas to that sensor electrode.
The single diffusion gap and measuring chamber of a conventional
sensor is thus replaced with a multitude of diffusion channels.
This essentially divides the pump cell surface into a multitude of
parallel operating pump cells. This reduces greatly the chance that
a single small particle covering the diffusion gap can render the
sensor inoperable. Because each pump cell part also has to operate
only on a much smaller gas sample compared to a regular wideband
sensor, its operating speed can be further increased. The
temperature of the sensor, which may be useful to measure because
of its effects on diffusion speed and Nernst voltage, can be
measured via the internal resistance (Ohmish impedance) of the
cell, as the material used for these sensors has a strong negative
temperature coefficient. The porous layer may be made of a material
that has strong thermal isolating characteristics. It can also
serve as protection for the pump cell itself. With appropriate
construction, this porous layer by itself can act as protection
shield for the sensor, thus eliminating the slowdown in response
speed caused by the flow dynamics of metallic shields around
conventional wideband sensors.
[0186] Accordingly, the apparatus, system, and method discussed
with reference to FIGS. 10-13 provide several advantages over
conventional ion concentration measuring systems. The elimination
of two devices provides for more easily manufactured sensors and
reduced delays within the sensors during use. More accurate results
are obtained due to the PWM scheme as compared to control loop
implementation. Since the internal resistance of the cell is
temperature dependent, the calculated internal resistance
R.sub.CELL value can then be used to measure temperature and to
regulate the temperature electrochemical cell (pump cell) by
controlling the pump cell heater.
[0187] FIG. 14 is a block diagram of an ion concentration sensor
1400 including a measuring chamber 1402, a primary electrochemical
cell system 1404 and a secondary electrochemical cell system 1406.
The ion concentration sensor 1400 may be used within various
systems and embodiments to measure the ion concentration of fluids
such as liquids and gases. As discussed below, the ion
concentration sensor 1400 may include pump cells, measuring cells,
and electrochemical cells to implement a nitrogen ion sensor.
[0188] The primary electrochemical system 1404 and the secondary
electrochemical system 1406 each include at least one
electrochemical cell and are configured to vary a flow of ions
1408, 1410 into the measuring chamber 1402 and out from the
measuring chamber 1402 in response to a corresponding pump current
1412, 1414 and to generate an output signal 1416, 1418 that is
based on ion concentrations 1420, 1422 within the measuring chamber
1402. Accordingly, the primary electrochemical system 1404 varies a
primary ion flow 1408 into the measuring chamber 1402 and out from
the measuring chamber 1402 in response to a primary pump current
1412. A first output signal 1416 is generated in accordance with
the first ion concentration 1420 within the measuring chamber 1402.
The secondary electrochemical system 1406 varies a secondary ion
flow 1410 into the measuring chamber 1402 and out from the
measuring chamber 1402 in response to a secondary pump current
1414. A second output signal 1418 is generated in accordance with
the second ion concentration 1422 within the measuring chamber.
Although the ion concentrations 1420, 1422 may be ion
concentrations of different elements, the ion concentrations 1420,
1422 are of the same element in the examples discussed herein.
Examples of output signals 1416, 1418 include electrochemical cell
voltages generated by an electrochemical cell or measuring cell.
For the examples discussed below, the second ion concentration 1422
is a local oxygen ion concentration near a catalytic electrode of
the secondary electrochemical system 1406 and the first ion
concentration 1420 is a substantially uniform oxygen ion
concentration throughout the measuring chamber other than near the
catalytic electrode (general oxygen ion concentration). For the
examples, therefore, the first ion concentration 1420 is within a
first region and the second ion concentration 1422 is within a
second region where the second region is closer to the secondary
electrochemical system than the primary electrochemical system. The
first and second ion concentrations 1420, 1422, however, may be ion
concentrations of different elements or have different
distributions within the measuring chamber 1402 depending on the
particular implementation.
[0189] During operation of the ion concentration sensor 1402, the
primary pump current 1412 is varied between a first constant
primary pump current and a second constant primary pump current to
vary the primary ion flow 1408. The varying ion flow increases and
decreases the first ion concentration 1420 resulting in a varying
first output signal 1426. The primary pump current 1412 is adjusted
in accordance with the first output signal 1416. For the
implementations below, the primary pump current 1412 is reversed
when the first output signal 1416 reaches an upper threshold and
when it reaches a lower threshold. The first constant primary pump
current has an opposite polarity from the second constant primary
pump current.
[0190] The secondary pump current 1414 is varied between a first
constant secondary pump current and a second constant secondary
pump current based on a relationship between the first output
signal 1416 and the second output signal 1418. For the
implementations below, the secondary pump current 1414 is reversed
when a difference between the first output signal 1416 and the
second output signal 1418 reaches an upper difference threshold and
when the difference reaches a lower difference threshold.
Accordingly, the second output signal 1418 indicates a relative ion
concentration between the first ion concentration 1420 and the
second ion concentration 1422 and the relationship between the
second output signal 1418 and the first output signal 1416
indicates an ion concentration. For the implementations below, for
example, the duty cycle of a control signal for controlling the
secondary pump current indicates an Oxides of Nitrogen (NOx)
concentration where the first output signal 1416 indicates oxygen
concentration and the second output signal 1418 indicates oxygen
concentration resulting from catalytically reducing NOx to oxygen
and nitrogen. Any signal or waveform related or derived from the
secondary pump current or the control signal for controlling the
current can be evaluated to determine the NOx concentration where
the evaluated waveform or signal depends on the particular
implementation. Also, a hysteresis function is formed that is
dependent on the thresholds and which can be adjusted by selecting
the thresholds.
[0191] FIG. 15 is a block diagram of an Oxides of Nitrogen (NOx)
sensor system 1500 including a NOx sensor 1501 having a pump cell
1502, an oxygen measuring cell 1504, and a nitrogen sensitive
electrochemical cell 1506. Accordingly, the NOx sensor 1501 is an
implementation of the ion sensor 1400 where the primary
electrochemical system 1404 includes the pump cell 1502 and the
oxygen measuring cell 1504 and where the secondary electrochemical
system 1406 includes the nitrogen sensitive electrochemical cell
1506. The pump cell 1502 is exposed to an external fluid 1508 that
is to be measured and a measuring chamber 1510. The oxygen
measuring cell 1504 and the nitrogen sensitive cell 1506 are
exposed to ambient fluid 1512 and the measuring chamber 1510.
Although the sensor 1501 may be designed to work with a variety of
fluids including gases and liquids, typical implementations include
configurations for measuring gas concentrations. An example of a
suitable implementation includes installing the NOx sensor 1501
within an exhaust system of a combustion engine in order to measure
the NOx concentration of exhaust gas. In such systems, therefore,
the external fluid 1508 is exhaust gas and the ambient fluid 1512
is ambient air.
[0192] A measured fluid 1514, such as a measured exhaust gas, is
received through a measuring opening such as diffusion gap or other
opening to the measuring chamber 1510. As described below with
reference to FIG. 16, the measured fluid 1514 is received through a
diffusion gap through the pump cell 1502. A sensor managing device
1516 manages the sensor 1501 by controlling cell currents and
detecting output signals. The oxygen measuring cell 1504 provides
an oxygen cell output signal 1518 that indicates the oxygen ion
concentration within the measuring chamber 1510. The oxygen ion
concentration, therefore, is an example of the first ion
concentration 1420 of the example of FIG. 14. The oxygen measuring
cell output signal 1518 is an example of the first output signal
1416.
[0193] During operation, the sensor managing device 1516 varies the
pump cell current 1520 through the pump cell 1502 between a first
constant pump current and a second constant pump current. The
oxygen measuring cell output signal 1518 is monitored by the sensor
managing device 1516 and used to determine when to switch the
direction of the pump current 1520. When the oxygen measuring cell
output signal 1518 reaches an upper threshold, the pump current
1520 is reversed from a positive constant pump current that pumps
oxygen ions into the measuring chamber 1510 to a negative constant
pump current that pumps oxygen ions out from the measuring chamber
1510. When the oxygen measuring cell output signal 1518 reaches a
lower threshold, the pump current 1520 is switched back to the
positive pump current. The lines illustrating the connections
between the sensor managing device and the sensor are functional
representations of signals and the actual number of physical
connections depends on the particular implementation. Some of the
connections may be to ground or other voltage potential. As
discussed with reference to FIG. 17, for example, an electrode from
each of the cells is connected to ground and only two electrical
connections (other than ground) are made between the sensor and the
sensor managing device.
[0194] The nitrogen sensitive electrochemical cell 1506 is any
electrochemical cell that reduces oxide of nitrogen (NOx) into
nitrogen and oxygen and that generates a nitrogen cell output
signal 1522 that indicates a local oxygen ion concentration at the
cell 1506. For the example discussed with reference to FIG. 16
below, the nitrogen sensitive electrochemical cell 1506 includes a
platinum and rhodium (Pt/Rh) electrode exposed to the measuring
chamber 1510 and that catalytically reduces the NOx to N.sub.2 and
O.sub.2. Where NOx is present, the local oxygen concentration at
the NOx cell electrode is higher than the oxygen concentration
within the rest of the measuring chamber 1510. This local oxygen
concentration, therefore, is an example of the second ion
concentration 1422 of FIG. 14. The sensor managing device 1516
compares the nitrogen cell output signal 1522 to the oxygen
measuring cell output signal 1518 and applies a nitrogen cell pump
current 1524 through the nitrogen sensitive electrochemical cell
1506. The nitrogen cell pump current 1524 pumps oxygen ions out
from the measuring chamber to the ambient air 1512 when the
nitrogen cell pump current 1524 is negative and pumps oxygen into
the measuring chamber 1510 from the ambient air 1512 when the
nitrogen cell pump current 1524 is positive. The nitrogen cell pump
current 1524 is reversed when the difference between the oxygen
cell output signal and the nitrogen cell output signal 1522 reaches
an upper threshold and when it reaches a lower threshold.
Accordingly, the function (wave form) of the control signal for
switching the nitrogen cell pump current 1524 indicates the NOx
concentration of the measured fluid 1514. The concentration may be
determined by observing other values or signals. The duty cycle of
the nitrogen cell pump current 1524 may be analyzed to determine
nitrogen concentration, for example. Accordingly, other values
derived or related to the difference between the oxygen measuring
cell output signal 1518 (first output signal 1416) and the nitrogen
sensitive cell output signal 1522 (second output signal 1418) may
be used to determine oxides of nitrogen concentration.
[0195] FIG. 16 is a block diagram of a cross section of a NOx
sensor 1600 for measuring ion concentrations in gases which is an
example of an implementation of the NOx sensor 1501 of FIG. 15.
Accordingly, the NOx sensor 1600 is an example of the NOx sensor
1501 where the NOx sensor 1600 can be used within the exhaust
system of a combustion engine. The sensor 1600 may also be used
within other systems and for other uses. Other uses may include
detection alarms and medical devices.
[0196] The NOx sensor 1600 includes two laminated zirconium dioxide
(ZrO2) ceramic layers 1602, 1604. A sealed spaced between the
layers 1602, 1604 forms the measuring chamber 1510. Platinum
electrodes 1606, 1608 are disposed on each side of a first layer
1602 such that an exhaust electrode 1606 is exposed to the exhaust
gas 1610 (external fluid 1508) and a measurement electrode 1608 is
exposed to the gases in the measuring chamber 1510. A hole 1612
within the first layer forms a diffusion gap for receiving a
measured gas 1614 (measured fluid 1514). Other types of diffusion
gaps and diffusion layers may be used in some circumstances.
[0197] A single platinum electrode (air electrode) 1616 is disposed
on a second ZrO2 layer 1604 such that the air electrode 1616 is
opposite the measuring chamber 1510 and exposed to ambient air 1618
(ambient fluid 1512). Another platinum electrode (oxygen measuring
electrode) 1620 is disposed on the second layer 1604 opposite the
air electrode 1616 to form the oxygen measuring cell 1504 with the
air electrode 1616 and the second layer 1604. A platinum/rhodium
(Pt/Rh) electrode 1622 forms the nitrogen sensitive electrochemical
cell 1506 with the air electrode 1616 and the second layer 1604.
Accordingly, the first layer 1602 with the platinum electrodes
1606, 1608 form the pump cell 1502 and the second layer 1604 with
the platinum layers 1616, 1620 and the Pt/Rh electrode 1622 form
the oxygen measuring cell 1504 and the nitrogen sensitive cell
1506. An electrical current 1520 through the pump cell 1502
transports oxygen ions in an opposite direction to the direction of
the electrical current. The voltage across the oxygen measuring
cell 1504, pump cell 1502, and the nitrogen sensitive cell 1506 is
governed by the Nernst equation:
Voutn=(R*T/4*F)*ln(Po1/Po2)
[0198] where R is the universal gas constant, T is the temperature
in degrees Kelvin, F is the Faraday constant, Po1 is the partial
Oxygen pressure of one electrode (air for the Oxygen measuring cell
and Pt/Rh cell, and exhaust gas for the pump cell), and Po2 is the
partial Oxygen pressure of the other electrode (electrodes inside
the measuring chamber). Air oxygen partial pressure is
approximately 20000 Pascals.
[0199] The nitrogen sensitive cell 1506 is responsive to nitrogen
and oxygen. The Pt/Rh electrode 1622 catalytically reduces oxides
of nitrogen (NOx) into Nitrogen (N2) and oxygen (O2). This
mechanism causes a local enrichment of oxygen content (partial
pressure) at the surface of the Pt/Rh electrode 1622. Accordingly,
the local concentration of oxygen near the Pt/Rh electrode
increases when NOx are present. The Nernst voltage between the air
electrode and the Pt/Rh electrode is lower than the oxygen
measuring cell output voltage (first output signal 1416). The local
oxygen concentration near the Pt/Rh electrode 1622 is an example of
the second ion concentration 1422 of FIG. 14.
[0200] FIG. 17 is a schematic function diagram of a NOx measuring
system 1700 including the sensor managing device 1516 connected to
the NOx sensor 1600 of FIG. 16. The NOx measuring system 1700 may
be implemented using any combination of hardware, software and
firmware. Various functions and operations of the functional blocks
described with reference to FIG. 17 may be implemented in any
number of devices, circuits or elements. Any of the functional
blocks may be integrated in a single device and the functions of
the blocks may be distributed over several devices, circuits and
elements. Some of the functional blocks may be omitted in some
instances. For example, the flip-flops may be omitted since the
operation of these devices is inherent in the counting of output
pulses by counters. FIG. 17, therefore, is a diagram of an example
of an implementation of the sensor 1600 as connected within a
measurement system. The various elements, devices, values, signals,
and functions described with reference to FIG. 17 may differ for
other measurement systems using other implementations of the
sensors 1400, 1501, 1600, depending on the particular structure,
use, environment and requirements of the particular measurement
system and sensor.
[0201] A first comparator 1702 with hysteresis continuously
compares the oxygen measuring cell voltage 1704 (first output
signal 1416) across the oxygen measuring cell 1504 to a reference
voltage 1706. For this example, the reference voltage 1706 is -450
mV. When the oxygen measuring cell voltage 1704 (first output
signal 1416) drops below a lower threshold, the first comparator
output is a logic high signal and when the oxygen measuring cell
voltage 1704 (first output signal 1416) rises above an upper
threshold, the first comparator output is a logic low signal. The
upper and lower thresholds are determined by the hysteresis of the
comparator 1702 and are selected to correspond to oxygen
concentration thresholds within the measuring chamber 1510. The
first comparator output 1707 is processed by a first flip-flop
(FF1) 1708 such that a clock input 1710 gates a FF1 output 1712 of
the first flip-flop 1708. The FF1 output 1712 controls a first
current switch 1714 which directs a pump cell current 1716 through
the pump cell 1502 between a first constant pump current 1718 and a
second constant pump current 1720. For this example, the first and
second constant pump currents are equal in magnitude but have an
opposite polarity. Therefore, after the first comparator output
1707 drops below the lower threshold (-1/2 hysteresis), the first
flip flop output 1712 is set to high at the next clock cycle. The
resulting high FF1 output sets the first current switch 1714 to
direct a constant positive pump current 1718 through pump cell
1502. In this state, the pump cell 1502 pumps oxygen from the
exhaust side electrode 1610 through the pump cell 1502 and into the
measuring chamber 1510. The first current switch 1714 remains in
this position until switched by the FF1 output 1712. When the
oxygen measuring cell voltage 1704 reaches the upper threshold
(+1/2 hysteresis), the first comparator 1702 generates a first
comparator output signal that is a logic low. At the next clock
cycle, the first flip-flop 1708 out changes to a logic low signal
to switch the first current switch 1714 to the negative constant
pump current 1720. FIG. 17 illustrates a switching current source
1724 that includes a positive current source 1726 and a negative
current source 1728 that are directed by the first switch 1714.
Accordingly, the switching current source 1724 provides the primary
pump current 1412 referred to in FIG. 14. Any of numerous
techniques and circuits can be used to achieve the functionality of
the switching current source 1724 that switches from the first
constant pump current 1718 to the second constant pump current
1720. An example of a suitable technique includes using an output
of an operational amplifier as an input to provide a feedback loop
as described above. Another example includes using a transistor
circuit as a current source and using an electronic switch to
switch between the current source and current sink.
[0202] The second comparator 1730 with hysteresis generates an
output 1732 based on a relationship between the oxygen measuring
cell voltage 1704 (first output signal 1416) and the Pt/Rh cell
output voltage 1734 (second output signal 1418). As described
above, the PT/Rh electrode 1622 reduces NOx into N2 and O2 and
generates and output 1734 based on the local oxygen concentration
near the Pt/Rh electrode 1622. When NOx are present, the local
oxygen concentration increases and the Nernst voltage between the
air electrode 1616 and the Pt/Rh electrode 1622 is lower than the
Nernst voltage between the air electrode 1622 and the measurement
electrode 1620 of the oxygen measuring cell 1504. The second
comparator 1730 continuously compares the Nernst voltage of the
oxygen measuring cell 1704 (first output signal) and the Nernst
voltage of the Pt/Rh cell (second output signal). When significant
levels of NOx are present and the difference of the PT/Rh cell
voltage and the oxygen measuring cell voltage is below a lower
threshold (e.g. the difference has magnitude above a threshold
magnitude), the second comparator 1730 generates a second
comparator output signal 1732 that is a logic low. At the next
clock cycle, a second flip-flop 1736 generates a FF2 output 1740
that is also a logic low. The FF2 low signal sets the second
current switch 1738 to direct the Pt/Rh cell current 1742
(secondary pump current 1414) through the Pt/Rh cell 1506 at a
first constant Pt/Rh cell current 1744 (first constant secondary
pump current). The first constant secondary pump current,
therefore, is a relatively small negative pump current that flows
from the air electrode 1616 to the Pt/Rh electrode 1622 for the
example of FIG. 17. When the first constant secondary pump current
is applied, the electrochemical cell forming the Pt/Rh cell 1506
functions as a pump cell to pump oxygen from the measuring chamber
1510 through the Pt/Rh cell 1506 to ambient air 1524. Accordingly,
oxygen flow through the Pt/Rh cell 1506 is opposite in direction to
the current flow 1742. When the oxygen is adequately depleted to
establish a local oxygen concentration that is less than a
threshold, the second comparator 1730 generates a second comparator
signal 1732 that is a logic high value. Accordingly, the comparator
1730 detects that the Pt/Rh cell and the oxygen measuring cell have
the same voltage (or nearly the same voltage due to the hysteresis)
and generates the high logic signal. At the next clock cycle, the
second flip-flop 1736 provides a logic high signal to the second
switch 1738 to switch the Pt/Rh cell current 1742 (secondary pump
current 1414) from the first constant Pt/Rh cell current 1744
(first constant secondary pump current) to a second constant Pt/Rh
cell current 1746 (second constant secondary pump current). The
Pt/Rh cell current 1742, therefore, switches from a constant
negative Pt/Rh cell current 1744 to a constant positive Pt/Rh cell
current 1746 where the currents 1744, 1746 are relatively small
compared to the pump cell currents 1718, 1720. The relatively small
positive current through the Pt/Rh cell pumps oxygen from the air
1524 into the measuring chamber 1510. The local oxygen
concentration near the Pt/Rh electrode continues to increase due to
the positive secondary pump current and the contribution of any
catalytically reduced NOx until the lower threshold is reached and
the process repeats. FIG. 17 illustrates a switching current source
1748 that includes a positive current source 1750 and a negative
current source 1752 that are directed by the second switch 1738.
Any of numerous techniques and circuits can be used to achieve the
functionality of the switching current source 1746 that switches
from the first constant Pt/Rh cell current 1744 to the second
constant Pt/Rh cell current 1746. Accordingly, the switching
current source 1748 provides the secondary pump current 1414
referred to in FIG. 14.
[0203] The NOx pulse density output of the second flip-flop output
signal 1740 indicates the concentration of NOX in the measuring
chamber 1510. Where there is no NOx present, the number of logic
highs (1s) and logic lows (0s) of the signal 1740 are equal for a
given time period. This occurs since there is no need to pump out
any additional oxygen produced by the Pt/Rh electrode 1622. As the
NOx concentration increases, more oxygen is catalytically produced
and pumped out and the number of 1s increases relative to the
number if 0s over a given time period. Accordingly, the duty cycle
of the NOx pulse density output 1740 is a measurement of NOx.
[0204] A computing device 1754 receives the control signal 1740 and
determines the NOx concentration from the duty cycle. The computing
device 1754 can be implementation using a processor, a
microprocessor, or combination of hardware, software and/or
firmware. The computing device may include counters in some
situations. In some cases, a table may be used to compare the duty
cycles to stored values correlated to concentrations. In other
situations, the concentration may be calculated based on a stored
equation. The relationship, for example, between the duty cycle and
concentration is linear, or nearly linear, in most situations.
Therefore, the detected duty cycle may be calculated and applied to
a linear equation to determine the concentration.
[0205] The various, signal polarities, magnitudes, logic levels and
frequencies are selected in accordance with the particular
implementation. Some examples of suitable values include the
following. An example of a suitable secondary pump current includes
a secondary pump current magnitude that is 2 to 3 times orders of
magnitude lower than then primary pump current. Such a selection is
often suitable since typical NOx content within the measuring
chamber is many orders lower than the oxygen content. The
hysteresis of the first comparator is chosen such that the
oscillation of the circuit controlled by the comparator is on the
order of 50 to 100 Hz. The hysteresis of the second comparator is
chosen such that the oscillation of the NOx circuit controlled by
the second comparator is on the order of one to two kilohertz (1-2
KHz). In many circumstances, the hysteresis of the second
comparator 1730 is selected to compensate for the voltage (V.sub.R)
across the Pt/Rh cell due to the resistance of the cell. The output
voltage (V.sub.Cell) of the Pt/Rh cell 1506 is due to the
combination of the Nernst voltage governed by the Nernst Equation
and the resistance voltage (V.sub.R). (V.sub.R) is equal to
(I.sub.Pump2) R.sub.Cell, where R.sub.Cell is the internal
resistance (impedance) of the Pt/Rh cell and where
(I.sub.Pump2).sub.is the secondary pump current. Although the
V.sub.R switches polarity depending on the current direction, the
Nernst voltage does not.
[0206] Therefore, the measuring system 1700 measures the NOx
content of exhaust gas 1508. A measured gas 1614 is diffused
through a single diffusion gap 1612 into the measuring chamber
1510. A primary electrochemical system directs and measures oxygen
within the measuring chamber while a secondary electrochemical
system operates in parallel to measure NOx. The secondary
electrochemical system reduced the NOx into N.sub.2 and O.sub.2 and
manages flow of O.sub.2 into and out from the measuring chamber in
parallel to the flow of oxygen managed by the primary
electrochemical system. As explained above, this mechanism provides
several advantages over conventional NOx sensors. For example,
conventional NOx sensors have less sensitivity and are more
susceptible to errors and noise due to their structure. As
discussed above, conventional NOx sensors typically include two
diffusion gaps and measuring chambers. A measured gas is received
by a wideband oxygen sensor through a first diffusion gap. The
wideband oxygen sensor is positioned in series (in terms of gas
flow) with a limit current flow sensor. The wideband oxygen sensor
is used to deplete the measured gas of oxygen while leaving NOx
intact. This oxygen depleted gas is diffused through the second
diffusion gap into second measuring chamber. A Pt/Rh cell within
the second chamber is oxygen and nitrogen sensitive. A constant
voltage imposed on this cell causes an oxygen ion flow. This
externally imposed voltage is opposed by the Nernst voltage of the
cell and, therefore, a very small current develops. The small
current is linear with the NOx content over a small range. The
range can be extended by imposing different voltages, at the cost
of resolution. Because this current is very small (in the
nano-ampere range) and reacts to NOx content very slowly, the
sensor follows the NOx content also very slowly. This is aggravated
by the need for the NOx to flow through two diffusion gaps and
measuring chambers. Because the system relies on a constant low
oxygen content, any error in that oxygen content shows up as error
in the NOx measurement. This of course limits the maximum
sensitivity of the sensor. The very small current is also very
sensitive to noise and the measurement is extremely sensitive to
electrical noise. These and other limitations are reduced or
eliminated with the structures, circuits, and techniques discussed
herein.
[0207] As mentioned above, the functions and elements described
with reference to FIG. 17 may be implemented using any combination
of hardware, software and/or firmware. An example of a suitable
implementation of the current source 1726 and current sink 1728
includes a current source with switchable polarity. A
Schmitt-Trigger circuit consisting of an operation amplifier and
resistors is an example of a suitable comparator 1702 with
Hysteresis. The voltage source can be realized with a resistor
divider network.
[0208] During operation, therefore, the comparator 1702 with
hysteresis compares the output of O.sub.2 measuring cell consisting
of the measuring chamber electrode 1620 and air electrode 1616 to a
fixed voltage from voltage source 1706. The output of the
comparator is synchronized to the clock source with the first
flip-flop (D-Flip Flop) 1708. The output of the flip flop 1708
controls the direction of the pump current through switch 1714. The
output of the D-Flip Flop 1708 is a pulse-density modulated signal
whose pulse density (or duty cycle) is proportional to the O.sub.2
concentration of the measured gas. The comparator (with Hysteresis)
1730 compares the output of the O.sub.2 reference cell (1620, 1616)
to the voltage at the Pt--Rh NOx sensitive cell consisting of
Pt--Rh electrode 1622 and air electrode 1616. If no NOx is present,
the Nernst voltages at the NOx sensitive electrode and the O.sub.2
sensitive only electrode should be the same. Due to the hysteresis
of the comparator 1730, however, a small pump current (I.sub.Pt/Rh)
is flowing through the Pt--Rh cell. This pump current, depending on
its direction, locally increases or decreases the oxygen content at
the Pt--Rh electrode. This changes the voltage at that electrode.
When one of the hysteresis points is reached, the output of the
comparator switches from 1 to 0 (or from 0 to 1). This switching is
clock synchronized by D-Flip-Flop 1736. The output of this
D-Flip-Flop in turn causes the polarity of pump current I.sub.Pt/Rh
to reverse direction via the electronic switch 1738. If no NOx is
present, no net O.sub.2 flow is needed through the Pt--Rh cell to
maintain the same Nernst voltage as the O.sub.2 reference cell. In
this case, for a given time period, the output of the D-Flip-Flop
1736 is high as often as it is low. As a result, the average duty
cycle of the NOx Pulse Density output is 50%. If NOx is present,
some of the NOx is decomposed at the Pt--Rh electrode to O2 and N2.
This causes a local excess of O2 molecules at the Pt--Rh electrode.
In order to maintain the same Nernst voltage between Pt--Rh cell
and O2 reference cell, this excess O2 must be removed. When current
flows through the Pt--Rh cell from the air electrode 1616 to Pt--Rh
electrode 1622, the cell behaves as an oxygen pump cell, pumping O2
ions from the Pt--Rh electrode to the air side. Therefore, for a
given time period in this scenario, the electronic switch 1738
spends more time in its active position where it is switched to
I.sub.Pt/Rh. As a result, the average duty cycle is greater than
50%. The duty cycle depends on the NOx concentration and the chosen
I.sub.Pt/Rh absolute value. In most circumstances, the relationship
between the average duty cycle and the NOx concentration of the
measurement gas is linear or nearly linear. In some situations, the
sensor system is calibrated to accurately correlate the duty cycle
to the NOx concentration. An example of a suitable calibration
technique includes adjusting one or more external resistors
implemented within the sensor unit. Such a technique may be
performed at the factory during manufacturing and establishes the
slope of the linear relationship. In some situations, calibration
may be omitted and determining whether calibration is needed
depends on manufacturing tolerances, required accuracy of the NOx
concentration measurement, and other factors.
[0209] The first comparator 1702 and flip-flop 1708 are an example
of a first detection circuit 1756 that is configured to detect a
first output signal generated by the primary electrochemical cell
system in accordance with a first ion concentration within the
measuring chamber and to control the first switching current
source. The first switching current source is configured to direct
the primary pump current through the primary electrochemical cell
system between a first constant primary pump current and a second
constant primary pump current to direct a first ion flow into a
measuring chamber and out from the measuring chamber. The primary
electrochemical system includes a measuring cell and a pump cell in
this example. The second comparator 1730 and the second flip flop
1738 are an example of a second detection circuit 1758 configured
to detect a second output signal generated by the secondary
electrochemical cell system in accordance with the second ion
concentration within the measuring chamber. The second detection
circuit 1758 is therefore configured to generate a control signal
1740 based on a relationship between the first output signal and
the second output signal. The second switching current source 1748
is configured to direct, in response to the control signal 1740,
the secondary pump current through the secondary electrochemical
cell system between the first constant secondary pump current and
the second constant secondary pump current to direct a second ion
flow into the measuring chamber and out from the measuring
chamber.
[0210] FIG. 18 is a graphical representation 1800 of an oxygen
concentration within the measuring chamber 1510. FIG. 18 shows
general relationships between values and the depicted amplitudes
and frequencies may not necessarily be to scale and may not depict
actual measured quantities. A first ion concentration curve 1802
represents a first ion concentration 1420 such as the general
oxygen concentration within the measuring chamber 1510. A second
ion concentration curve 1804 represents a second ion concentration
1422 such as the local oxygen concentration near the Pt/Rh
electrode 1622. In most situations, the curves 1802, 1804 are
proportional to the primary and second output signals 1416, 1418
such as the cell voltages of the oxygen measuring cell 1504 and the
nitrogen sensing cell 1506. The first ion concentration curve 1802
varies between an oxygen concentration upper threshold 1806 and an
oxygen concentration lower threshold 1808 although the thresholds
may be exceeded in some circumstances. The oxygen concentration
lower threshold 1808 may be equal to, or very near, zero in some
situations. The oxygen concentration lower threshold 1808,
therefore, corresponds to the lower threshold applied by the first
comparator when comparing the oxygen measuring cell voltage 1704 to
the reference voltage 1706. The oxygen concentration upper
threshold 1806 corresponds to the upper threshold applied by the
first comparator 1702.
[0211] FIG. 19 is a graphical representation 1900 of a NOx pulse
output curve 1902. FIG. 19 shows general relationships between
values and the depicted amplitudes and frequencies may not
necessarily be to scale and may not depict actual measured
quantities. The NOx pulse output curve 1902 varies between logic
high 1904 and logic low 1906 levels and has a duty cycle dependent
on the NOx concentration within the measuring chamber 1510. The
second flip-flop output 1740 may provide the NOx pulse output curve
1902. A first portion 1906 of the NOx pulse output curve 1902
represents the output of the second flip flop 1736 when no NOx are
present in the measuring chamber. For the first portion 1906 the
duty cycle is 50 percent. A second portion 1908 represents the
second flip-flop output 1740 when NOx are present. The second
portion has a duty cycle that is greater than 50 percent since, at
a given clock rate, the number of 1s is greater than the number of
0s within a time period.
[0212] FIG. 20 is a block diagram of a sensor system 2000 including
a NOx sensor 2002 connected to a sensor managing device 2004 where
the NOx sensor 2002 includes a single oxygen electrochemical cell
2006 for performing the functions of the oxygen sensor cell and the
oxygen measuring cell. Accordingly, the NOx sensor 2002 is an
implementation of the ion sensor 1400 where the primary
electrochemical system 1404 includes the oxygen electrochemical
cell 2006 and where the secondary electrochemical system 1406
includes the nitrogen sensitive electrochemical cell 2008. An
external fluid 2010 is received through a diffusion gap, diffusion
layer, or other opening into the measuring chamber 2012 as a
measured fluid 2014. The oxygen electrochemical cell 2006 and the
nitrogen sensitive electrochemical cell 2008 are exposed to ambient
fluid 2016 and the measuring chamber 2012. Although the sensor 2002
may be designed to work with a variety of fluids including gases
and liquids, typical implementations include configurations for
measuring gas concentrations. An example of a suitable
implementation includes installing the NOx sensor 2002 within an
exhaust system of a combustion engine in order to measure the NOx
concentration of exhaust gas. In such systems, therefore, the
external fluid 2010 is an exhaust gas and the ambient fluid 2014 is
ambient air. The sensor system 2000 may be implemented using any
combination of hardware, software and firmware. Various functions
and operations of the functional blocks described herein may be
implemented in any number of devices, circuits or elements. Any of
the functional blocks may be integrated in a single device and the
functions of the blocks may be distributed over several devices,
circuits and elements.
[0213] A sensor measuring device 2004 varies a pump current 2018
through the oxygen electrochemical cell 2006 between a first
constant current and a second constant current in accordance with a
cell voltage 2020 at the oxygen electrochemical cell 2006. The
oxygen electrochemical cell 2006 moves ions between the measuring
chamber 2012 and an ambient fluid 2016, such as air, based on the
pump current 2018 flowing through the oxygen electrochemical cell
2006. The sensor measuring device 2004 also varies a pump current
2022 through the nitrogen sensitive electrochemical cell 2008
between a first constant current and a second constant current in
accordance with a relationship between the cell voltage 2020 at the
oxygen electrochemical cell 2006 and the cell voltage 2024 at the
nitrogen sensitive cell 2008. The nitrogen electrochemical cell
2006 reduces NOx into oxygen ions and nitrogen ions and moves the
oxygen ions between the measuring chamber 2012 and an ambient fluid
2016, such as air, based on the pump current 2022 flowing through
the nitrogen sensitive electrochemical cell 2008.
[0214] The sensor measuring device 2004 extracts the Nernst voltage
of the oxygen electrochemical cell 2006 from the total cell voltage
2020 in order to provide a reference for comparison to the cell
voltage 2024 of the nitrogen sensitive electrochemical cell 2008.
The internal resistance of the oxygen electrochemical cell is
determined and subtracted from the cell voltage to obtain the
Nernst voltage of the oxygen electrochemical cell 2006 which
indicates the oxygen ion concentration within the first region of
the measured fluid within the measuring chamber. The Nernst voltage
of the oxygen electrochemical cell 2006, therefore, is an example
of the first output signal 1416 of FIG. 14.
[0215] During operation, the sensor managing device 2004 varies the
cell current 2012 through the oxygen electrochemical cell 2006
between a first constant pump current and a second constant pump
current. The oxygen measuring cell output signal 2020 is monitored
by the sensor managing device 2004 and used to determine when to
switch the direction of the cell current 2018. When the oxygen
measuring cell output signal 2020 reaches an upper threshold, the
cell current 2018 is reversed from a positive constant pump current
that pumps oxygen ions into the measuring chamber 2012 to a
negative constant pump current that pumps oxygen ions out from the
measuring chamber 2012. When the oxygen measuring cell output
signal 2020 reaches a lower threshold, the cell current 2018 is
switched back to the positive pump current. The lines illustrating
the connections between the sensor managing device 2004 and the
sensor are functional representations of signals and the actual
number of physical connections depends on the particular
implementation. Some of the connections may be to ground or other
voltage potential. As discussed with reference to FIG. 22, for
example, the air electrode is connected to ground and only two
electrical connections (other than ground) are made between the
sensor and the sensor managing device.
[0216] The nitrogen sensitive electrochemical cell 2008 is any
electrochemical cell that reduces oxide of nitrogen (NOx) into
nitrogen and oxygen and that generates a nitrogen cell output
signal 2024 that indicates a local oxygen ion concentration at the
cell 2008. For the example discussed with reference to FIG. 21
below, the nitrogen sensitive electrochemical cell 2008 includes a
platinum and rhodium (Pt/Rh) electrode exposed to the measuring
chamber 2012 that catalytically reduces the NOx to N.sub.2 and
O.sub.2. Where NOx is present, the local oxygen concentration at
the NOx cell electrode is higher than the oxygen concentration
within the rest of the measuring chamber 2012. This local oxygen
concentration, therefore, is an example of the second ion
concentration 1422 of FIG. 14. The sensor managing device 2004
compares the nitrogen cell output signal 2024 to the Nernst
contribution of the oxygen measuring cell output signal 2020 and
applies a nitrogen cell pump current 2022 through the nitrogen
sensitive electrochemical cell 2008. The nitrogen cell pump current
2022 pumps oxygen ions out from the measuring chamber to the
ambient air 2014 when the nitrogen cell pump current 2022 is
negative and pumps oxygen into the measuring chamber 2012 from the
ambient air 2012 when the nitrogen cell pump current 2022 is
positive. The nitrogen cell pump current 2022 is reversed when the
difference between the Nernst portion of the oxygen cell output
signal and the nitrogen cell output signal 2024 reaches an upper
threshold and when it reaches a lower threshold. Accordingly, the
function (wave form) of the control signal for switching the
nitrogen cell pump current 2022 indicates the NOx concentration of
the measured fluid 2014. The concentration may be determined by
observing other values or signals. The duty cycle of the nitrogen
cell pump current 1524 may be analyzed to determine nitrogen
concentration, for example. Accordingly, any value derived or
related to the difference between the oxygen measuring cell output
signal 2020 (first output signal 1416) and the nitrogen sensitive
cell output signal 2024 (second output signal 1418) may be used to
determine nitrogen concentration.
[0217] As compared to the NOx measuring system of FIG. 15, the NOx
measuring system 2000 described with reference to FIG. 20 uses a
single electrochemical cell for pumping and measuring ions rather
then separate cells. As mentioned above, the Nernst voltage must be
extracted from the total cell voltage of the in order to provide a
reference for determining the NOx concentration.
[0218] As described above, the electrochemical cell has an internal
resistance. When a constant electrical current is forced through
the oxygen electrochemical cell, a voltage is created at the cell
which is the sum of the Nernst voltage and the voltage drop
(resistance voltage) created by the internal resistance of the
electrochemical cell. The internal resistance is the real impedance
of the cell sometimes referred to as the Ohmish impedance. The
resistance voltage (V.sub.R) results from the pump current flowing
through the internal resistance. A Nernst voltage indicates the
oxygen concentration in the measuring chamber and is equal to the
difference between the total electrochemical cell voltage
(V.sub.CELL) and the resistance voltage (V.sub.R). The Nernst
voltage, therefore, can be calculated by subtracting the resistance
voltage (V.sub.R) from the electrochemical voltage (V.sub.CELL). In
the example of FIG. 20, the sensor managing device 2004 continually
switches the pump current 2018 between positive and negative
constant currents, measures the cell voltage, and determines the
oxygen concentration based on the Nernst voltage by subtracting the
resistance voltage (V.sub.R). The sensor managing device 2004 may
include a current managing unit and a computing device where the
current managing unit controls the current flow and measures the
cell voltage.
[0219] Accordingly, the measured voltage of the oxygen
electrochemical cell is used to trigger the reversal of pump
current through the oxygen electrochemical cell. For example, if
during a positive pump current (I.sub.P) the Nernst voltage
(|V.sub.CELL|-|R*I.sub.P|)>=0.5 Volt, then the pump current
(I.sub.P) is reversed, R (internal resistance) is calculated as
described below, and the process continues with a negative constant
pump current until the Nernst Voltage is <=0.4V. Then the pump
current (I.sub.P) is polarized back to positive and so on. In this
example, the Hysteresis voltage is 0.1V (0.5V-0.4V). Different
Hysteresis Voltages can be used.
[0220] An example of a suitable technique of determining the
internal resistance (R) of the oxygen electrochemical cell includes
measuring the voltage change at the cell at the transition point
between positive and negative current and/or between negative and
positive current through the cell. Since the pump current is
switched between constant positive and negative currents, the
resistance is calculated based on Ohms law.
[0221] The internal resistance is dependent on the temperature of
the electrochemical cell. At a polarity reversal of the pump
current, the cell has not had time to react and has not pumped any
oxygen in the new direction. So the oxygen concentration
difference, which determines the Nernst voltage, has not yet
changed by a significant amount. Accordingly, the voltage change at
the cell is at least mostly caused by the change in current. Based
on the difference in current and difference in voltage, the
internal resistance is determined based on the relationship
R.sub.CELL=.DELTA.V.sub.CELL/.DELTA.I.sub.P, where
.DELTA.V.sub.CELL is the difference in voltage at the cell and
.DELTA.IP is the difference in pump current. The internal
resistance R.sub.CELL is used to determine the voltage drop
(V.sub.R) due to the internal resistance R.sub.CELL based on the
Ohms Law, V.sub.R=R.sub.CELL*I.sub.P. The resistance voltage
(V.sub.R) is subtracted from the actual voltage (V.sub.CELL) on the
oxygen electrochemical cell to provide the reference for comparing
to the nitrogen cell output 2024. In most applications, the voltage
change .DELTA.V.sub.CELL can be measured up to a few microseconds
before and after the current transition. The difference between the
voltage immediately before the polarity reversal and immediately
after is the .DELTA.V.sub.CELL voltage change. An example of a
suitable technique for measuring the .DELTA.V.sub.CELL voltage
includes using a sample and hold circuit.
[0222] An example of suitable sensor managing device 2004 includes
an apparatus that is configured to connect to the measuring cell
system (oxygen electrochemical cell 2006 and the nitrogen sensitive
electrochemical cell 2008) through an interface 2026 and includes
circuitry forming a current managing unit and a computing device.
The interface 2026 may include an electrical connector, direct
cable connection, or other electrical contact arrangement for
conveying signals between the sensor 2002 and the sensor managing
device 2004.
[0223] FIG. 21 is a block diagram of a cross section of a NOx
sensor 2100 for measuring ion concentrations in gases which is an
example of an implementation of the NOx sensor 2002 of FIG. 20.
Accordingly, the NOx sensor 2100 is an example of the NOx sensor
2002 where the NOx sensor 2100 can be used within the exhaust
system of a combustion engine. The sensor 2100 may also be used
within other systems and for other uses. Examples of other uses
include detection alarms and medical devices.
[0224] The NOx sensor 2100 includes a single laminated zirconium
dioxide (ZrO2) ceramic layer 2102. A space between the layer 2102
and a chamber housing 2104 forms the measuring chamber 2012. A hole
2106 within the chamber housing 2104 forms a diffusion gap for
receiving a measured gas 2108 (measured fluid 2014). The diffusion
gap may be a diffusion gap, diffusion layer, or other opening into
the measuring chamber.
[0225] A single platinum electrode (air electrode) 2110 is disposed
on the ZrO2 layer 2102 such that the air electrode 2110 is opposite
the measuring chamber 2012 and exposed to ambient air 2112 (ambient
fluid 2014). Another platinum electrode (oxygen electrode) 2114 is
disposed on the layer 2102 opposite the air electrode 2110 and is
exposed to the measured gas 2108 with the measuring chamber to form
the oxygen electrochemical cell 2006 with the air electrode 2110
and the ZrO2 layer 2102. A platinum/rhodium (Pt/Rh) electrode 2116
is disposed on the measuring chamber side of the ZrO2 layer 2102
and is also exposed to the measured gas 2108 within the measuring
chamber 2012. The platinum/rhodium (Pt/Rh) electrode 2116 forms the
nitrogen sensitive electrochemical cell 2008 with the air electrode
2110 and the ZrO2 layer 2102. Accordingly, the oxygen electrode
2114, air electrode 2110, and ZrO2 layer 2102 form the oxygen
electrochemical cell 2006 and the Pt/Rh electrode 2116, air
electrode 2110, and ZrO2 layer 2102 form the nitrogen sensitive
cell 2008.
[0226] An electrical current 2018 through the oxygen
electrochemical cell 2006 transports oxygen ions in an opposite
direction to the direction of the electrical current. The nitrogen
sensitive cell 2008 is responsive to nitrogen and oxygen. The Pt/Rh
electrode 2116 catalytically reduces oxides of nitrogen (NOx) into
Nitrogen (N2) and oxygen (O2). This mechanism causes a local
enrichment of oxygen content (partial pressure) at the surface of
the Pt/Rh electrode 2116. Accordingly, the local concentration of
oxygen near the Pt/Rh electrode 2116 increases when NOx are
present. The Nernst voltage between the air electrode 2110 and the
Pt/Rh electrode 2116 is lower than the oxygen measuring cell output
voltage 2020 (first output signal 1416) when NOx are present. The
local oxygen concentration near the Pt/Rh electrode 2116 is an
example of the second ion concentration 1422 of FIG. 14.
[0227] FIG. 22 is a schematic function diagram of a NOx measuring
system 2200 including the sensor managing device 2004 connected to
the NOx sensor 2100 of FIG. 21. The NOx measuring system 2200 may
be implemented using any combination of hardware, software and
firmware. Various functions and operations of the functional blocks
described with reference to FIG. 22 may be implemented in any
number of devices, circuits or elements. Any of the functional
blocks may be integrated in a single device and the functions of
the blocks may be distributed over several devices, circuits and
elements. Some of the functional blocks may be omitted in some
instances. For example, the flip-flops may be omitted since the
operation of these devices is inherent in the counting of output
pulses by the counters. FIG. 22, therefore, is a diagram of an
example of an implementation of the sensor 2100 as connected within
a measurement system. The various elements, devices, values,
signals, and functions described with reference to FIG. 22 may
differ for other measurement systems using other implementations of
the sensors 1400, 2002, 2100, depending on the particular
structure, use, environment and requirements of the particular
measurement system and sensor.
[0228] A first comparator 2202 with hysteresis continuously
compares the oxygen measuring cell voltage 2204 across the oxygen
electrochemical cell 2006 to a reference voltage 2206. For this
example, the reference voltage 2206 is -450 mV. When the oxygen
measuring cell voltage 2204 drops below a lower threshold, the
first comparator output 2207 is a logic high signal and when the
oxygen measuring cell voltage 2020 rises above an upper threshold,
the first comparator output is a logic low signal. The upper and
lower thresholds are determined by the hysteresis of the comparator
and are selected to correspond to oxygen concentration thresholds
within the measuring chamber 2012. The thresholds also take into
account the contribution of the voltage due to the internal
resistance of the oxygen electrochemical cell. Accordingly, the
hysteresis of the first comparator 2202 in the system of FIG. 22 is
larger than the hysteresis of the first comparator 1702 of the
system 1700 discussed with reference to FIG. 17.
[0229] The first comparator output 2207 is processed by a first
flip-flop (FF1) 2208 such that a clock input 2210 gates a FF1
output 2212 of the first flip-flop 2208. The FF1 output 2212
controls a first current switch 2214 which directs a oxygen cell
pump current 2216 through the oxygen electrochemical cell 2006
between a first constant pump current 2218 and a second constant
pump current 2220. For this example, the first and second constant
pump currents are equal in magnitude but have an opposite
polarity.
[0230] Therefore, after the first comparator output drops below the
lower threshold (-1/2 hysteresis), the first flip flop output 2212
is set to high at the next clock cycle. The resulting high FF1
output sets the first current switch 2214 to direct a constant
positive pump current 2218 through the oxygen electrochemical cell
2006. In this state, the electrochemical cell 2006 pumps oxygen
from the oxygen electrode 2114 through the oxygen electrochemical
cell 2006 and into the measuring chamber 2012 from the ambient air.
The first current switch 2214 remains in this position until
switched by the FF1 output 2212. When the oxygen measuring cell
voltage 2204 reaches the upper threshold (+1/2 hysteresis), the
first comparator 2202 generates a first comparator output signal
that is a logic low. At the next clock cycle, the first flip-flop
2208 output changes to a logic low signal to switch the first
current switch 2214 to the negative constant pump current 2220. In
this state, oxygen is pump out from the measuring chamber 2012 to
ambient air. FIG. 22 illustrates a switching current source 2224
that includes a positive current source 2226 and a negative current
source 2228 that are directed by the first switch 2214. Any of
numerous techniques and circuits can be used to achieve the
functionality of the switching current source 2224 that switches
from the first constant pump current 2218 to the second constant
pump current 2220. Accordingly, for this example, the switching
current source 2224 provides the primary pump current 1412 referred
to in FIG. 14.
[0231] The second comparator 2230 with hysteresis generates an
output 2232 based on a relationship between the Nernst voltage
portion 2233 of the oxygen cell voltage 2024 (first output signal
1416) and the Pt/Rh cell output voltage 2234 (second output signal
1418). The second comparator 2230, therefore, compares the Nernst
voltage produced by the oxygen electrochemical cell 2006 and not
the total voltage across the cell in this example. A Nernst voltage
extractor 2235 extracts the Nernst voltage from the total voltage
of the oxygen electrochemical cell 2006. Accordingly, the Nernst
voltage extracted by the Nernst voltage extractor 2235 is an
example of the first output signal 1416 discussed with reference to
FIG. 14. An example of suitable implementation of the Nernst
voltage extractor 2035 includes a double sample circuit that
samples the O2 reference voltage immediately prior to a polarity
reversal of the pump current and immediately after a reversal. The
difference between the two sample points is twice as large as the
voltage caused by the pump current through the internal resistance
of the cell.
[0232] The Nernst voltage extractor 2235 may be implemented using
any of numerous techniques. An example of a suitable implementation
includes using a sample-and-hold circuit. Sample-and-Hold circuits
are typically implemented as part of analog to digital converter
designs and consist of a capacitor, an electronic switch, and a low
output impedance amplifier, such as an op-amp. A short sample pulse
switches the switch on, which connects the capacitor to the
amplifier output. This charges the capacitor to the output voltage.
When the pulse disappears, the capacitor holds that charge to be
read by the A/D conversion. In the Nernst extractor, the
sample-and-hold capacitor is charged (sampled) with a very short
pulse after the polarity of the current is reversed. At that time,
the Nernst voltage has not had time to change. The total voltage at
the cell is the sum of the Nernst Voltage (V.sub.nernst) and the
voltage caused by the pump current (I.sub.pump) and the internal
impedance of the cell (V.sub.icell). V.sub.icell is, by Ohms law,
I.sub.pump*R.sub.icell, where R.sub.icell is the cells impedance.
As an example: if the current switched from positive to negative
and, as a result, the total cell output voltage increases to reach
an upper threshold voltage. Measuring the difference between upper
threshold voltage and the voltage sampled immediately after the
switch provides the information to calculate V.sub.icell and
therefore R.sub.icell, because the magnitude of I.sub.pump is known
and fixed. The voltage at the cell before the switch (V.sub.upperh)
is V.sub.nernstupper+I.sub.pump*R.sub.icell. The voltage at the
capacitor after sampling right after the polarity reversal is
V.sub.sample=V.sub.nernst-I.sub.pump*R.sub.icell because the Nernst
voltage V.sub.nernst did not have time to change in the very short
time interval (microseconds) between polarity reversal of the
current and sampling of the new cell voltage. Therefore,
V.sub.upperh-V.sub.sample=2*I.sub.pump*R.sub.icell. This
subtraction can be performed by a differential amplifier and does
not need to be done digitally, although such an implementation may
be preferred in some situations. Dividing this difference by 2,
using a voltage divider, for example, yields V.sub.icell, because
V.sub.icell=I.sub.pump*R.sub.icell. The new lower threshold for the
comparison of the cell voltage is
V.sub.lowerh=V.sub.nernstlower-V.sub.icell. Because I.sub.pump is
constant, V.sub.icell changes only with the impedance R.sub.icell,
which is temperature dependent. Thus, V.sub.icell can also be used
for temperature regulation.
[0233] V.sub.nernstupper and V.sub.nernstlower are design
parameters that are selected in accordance with the particular
implementation and requirements. The parameters may be the same
value in some circumstances. The additions and subtractions in
these voltage calculations can be done digitally, or simpler with
simple analog circuits such as differential and summing amplifiers
using op-amps.
[0234] As described above, the PT/Rh electrode 2116 reduces NOx
into N2 and O2 and generates and output 2024 based on the local
oxygen concentration near the Pt/Rh electrode 2116. When NOx are
present, the local oxygen concentration increases and the Nernst
voltage between the air electrode 2110 and the Pt/Rh electrode 2116
is lower than then the Nernst voltage between the air electrode
2110 and the oxygen electrode 2114 of the oxygen electrochemical
cell 2006. The second comparator 2230 continuously compares the
Nernst voltage of the oxygen measuring cell 2006 (first output
signal) and the nitrogen sensitive cell output 2024 (second output
signal). Although there is a small contribution from the internal
resistance, the current used in the nitrogen sensitive cell is a
few orders of magnitude lower than the main oxygen cell pump
current and, therefore, the voltage change is significantly small
compared to the Nernst voltage. As a result a Nernst voltage
extraction is not necessary for the nitrogen sensitive cell in most
circumstances. The nitrogen sensitive cell can operate with fixed
upper and lower thresholds that take the V.sub.icell into account.
Since the temperature (and therefore R.sub.icell) is typically
regulated, R.sub.icell does not vary significantly.
[0235] When significant levels of NOx are present and the
difference of the nitrogen sensitive cell voltage 2024 and the
oxygen electrochemical cell voltage 2020 is below a lower threshold
(e.g. the difference has magnitude above a threshold magnitude),
the second comparator 2230 generates a second comparator output
signal 2232 that is a logic low level. At the next clock cycle, a
second flip-flop 2236 generates a FF2 output 1740 that is also a
logic low level. The FF2 low signal sets the second current switch
2238 to direct the nitrogen sensitive cell current 2022 (secondary
pump current 1414) through the nitrogen sensitive cell 2008 at a
first constant nitrogen cell current 2244 (first constant secondary
pump current). The first constant secondary pump current,
therefore, is a relatively small negative pump current that flows
from the air electrode 2110 to the Pt/Rh electrode 2116 for the
example of FIG. 22. When the first constant secondary pump current
is applied, the electrochemical cell forming the nitrogen sensitive
cell 2008 functions as a pump cell to pump oxygen from the
measuring chamber 2012 through the nitrogen sensitive cell 2008 to
ambient air 2014. Accordingly, oxygen flow through the nitrogen
sensitive cell 2008 is opposite in direction to the current flow
2042. When the oxygen is adequately depleted to establish a local
oxygen concentration that is less than a threshold, the second
comparator 2230 generates a second comparator signal 2232 that is a
logic high level. Accordingly, the comparator 2230 detects that the
nitrogen sensitive cell voltage and the Nernst portion 2233 of
voltage across the oxygen measuring cell are the same voltage (or
nearly the same voltage due to the hysteresis) and generates the
high logic signal. At the next clock cycle, the second flip-flop
2236 provides a logic high signal to the second switch 2238 to
switch the nitrogen sensitive cell current 2242 (secondary pump
current 1414) from the first constant nitrogen sensitive cell
current 2244 (first constant secondary pump current) to a second
constant nitrogen sensitive cell current 2246 (second constant
secondary pump current). The nitrogen sensitive cell current 2242,
therefore, switches from a constant negative nitrogen sensitive
cell current 2244 to a constant positive nitrogen sensitive cell
current 2246 where the currents 2244, 2246 are relatively small
compared to the pump cell currents 2218, 2220 through the oxygen
electrochemical cell 2006. The relatively small positive current
through the nitrogen sensitive cell 2008 pumps oxygen from the air
2014 into the measuring chamber 2012. The local oxygen
concentration near the Pt/Rh electrode 2116 continues to increase
due to the positive secondary pump current and the contribution of
any catalytically reduced NOx until the lower threshold is reached
and the process repeats. FIG. 22 illustrates a switching current
source 2248 that includes a positive current source 2250 and a
negative current source 2252 that are directed by the second switch
2238. Any of numerous techniques and circuits can be used to
achieve the functionality of the switching current source 2246 that
switches from the first constant nitrogen sensitive cell current
2244 to the second constant nitrogen sensitive cell current 2246.
Accordingly, the switching current source 2248 provides the
secondary pump current 1414 referred to in FIG. 14.
[0236] The NOx pulse density output of the second flip-flop output
signal 2240 indicates the concentration of NOx in the measuring
chamber 2012. Where there is no NOx present, the number of logic
highs (1s) and logic lows (0s) of the signal 2240 are equal for a
given time period. This occurs since there is no need to pump out
any additional oxygen produced by the Pt/Rh electrode 2116. As the
NOx concentration increases, more oxygen is catalytically produced
and pumped out and the number of 1s increases relative to the
number if 0s over a given time period. Accordingly, the duty cycle
of the NOx pulse density output 2240 is a measurement of NOx.
[0237] The various, signal polarities, magnitudes, logic levels and
frequencies are selected in accordance with the particular
implementation. Some examples of suitable values include the
following. An example of a suitable secondary pump current includes
a secondary pump current magnitude that is 2 to 3 times orders of
magnitude lower than then primary pump current. Such a selection is
often suitable since typical NOx content within the measuring
chamber 2012 is many orders lower than the oxygen content. The
hysteresis of the first comparator 2202 is chosen such that the
oscillation of the circuit controlled by the comparator 2202 is on
the order of 50 to 100 Hz. The hysteresis of the second comparator
2230 is chosen such that the oscillation of the NOx circuit
controlled by the second comparator 2230 is on the order of one to
two kilohertz (1-2 KHz). As explained above, the hysteresis of the
second comparator 2230 is selected to compensate for the voltage
(V.sub.R) across the Pt/Rh cell due to the resistance of the cell.
Since I.sub.pump2 is a few orders of magnitude smaller than
I.sub.pump1, V.sub.R is also much smaller. This contribution is
small enough that the upper and lower threshold of the Nernst
voltage comparison can be adjusted by adding R.sub.cell*I.sub.pump2
to the upper threshold of comparison and subtracting
R.sub.cell*I.sub.pump2 from the lower threshold. With typical
R.sub.cell of about 80 Ohms and I.sub.pump2 on the order of 10
micro-Amps, the adjustment is approximately 0.8 mV and, therefore,
relatively small.
[0238] Therefore, the measuring system 2200 measures the NOx
content of an exhaust gas 2010. A measured gas 21080 is diffused
through a single diffusion gap 2106 into the measuring chamber
2012. A primary electrochemical system directs and measures oxygen
within the measuring chamber 2012 while a secondary electrochemical
system operates in parallel to measure NOx. The secondary
electrochemical system reduces the NOx into N2 and O2 and manages
flow of O2 into and out from the measuring chamber 2012 in parallel
to the flow of oxygen managed by the primary electrochemical
system. This mechanism provides several advantages over
conventional NOx sensors as discussed above. The first comparator
2202 and flip-flop 2208 are an example of a first detection circuit
2256 that is configured to detect a first output signal generated
by the primary electrochemical cell system in accordance with a
first ion concentration within the measuring chamber and to control
the first switching current source 2224. The first switching
current source 2224 is configured to direct the primary pump
current through the primary electrochemical cell system between a
first constant primary pump current and a second constant primary
pump current to direct a first ion flow into a measuring chamber
and out from the measuring chamber. The primary electrochemical
cell system is a single electrochemical cell in this example that
produces a single cell output. The second comparator 2230 and the
second flip flop 2236 are an example of a second detection circuit
2258 configured to detect a second output signal generated by the
secondary electrochemical cell system in accordance with the second
ion concentration within the measuring chamber. The second
detection circuit 2258 is therefore configured to generate a
control signal 2240 based on a relationship between the first
output signal and the second output signal. The second switching
current source 2248 is configured to direct, in response to the
control signal 2240, the secondary pump current through the
secondary electrochemical cell system between the first constant
secondary pump current and the second constant secondary pump
current to direct a second ion flow into the measuring chamber and
out from the measuring chamber.
[0239] FIG. 23 is flow chart of a method of managing a sensor
having a primary electrochemical cell system and a secondary
electrochemical cell system. Although the method may be performed
by any combination of hardware, software, and/or firmware, the
method is performed in a sensor managing device in this
example.
[0240] At step 2302, the first output signal is detected. The first
output signal is generated by the primary electrochemical cell
system in accordance with the first ion concentration within the
measuring chamber. The first comparator receives the first output
signal and compares it to a reference voltage to generate a control
signal for controlling the first switching current source. Where
the primary electrochemical cell system includes a separate pump
cell and measuring cell, the first output signal is generated by
the measuring cell. Where the primary electrochemical cell system
includes a single electrochemical cell, the first output signal is
single cell output voltage generated by the single electrochemical
cell.
[0241] At step 2304, the primary pump current is directed through
the primary electrochemical cell system between the first constant
primary pump current and the second constant primary pump current.
The first switching current source directs the current in response
to the control signal produced by the first comparator and
"clocked" by the first flip flop. The varying current directs a
first ion flow into the measuring chamber and out from the
measuring chamber. Where the primary electrochemical cell system
includes a separate pump cell and measuring cell, the primary pump
current is the current through the pump cell. Where the primary
electrochemical cell system includes a single electrochemical cell,
the primary pump current is the current through the single
electrochemical cell.
[0242] At step 2306, the second output signal is detected. The
second output signal is generated by the secondary electrochemical
cell system in accordance with the second ion concentration within
the measuring chamber. The second comparator receives the second
output signal and compares it to either the first output signal or
to Nernst voltage portion of the first output signal to generate a
control signal for controlling the second switching current source.
Where the primary electrochemical cell system includes a separate
pump cell and measuring cell, the second output signal is compared
to the first output signal. Where the primary electrochemical cell
system includes a single electrochemical cell, the second output
signal is compared to the Nernst portion of the first output signal
which is provided by the Nernst voltage extractor.
[0243] At step 2308, the secondary pump current is directed through
the secondary electrochemical cell system based on a relationship
between the first output signal and the second output signal. The
secondary pump current is varied between the first constant
secondary pump current and the second constant secondary pump
current. The second switching current source directs the current in
response to the control signal produced by the second comparator
and "clocked" by the second flip flop. The varying current directs
a second ion flow into the measuring chamber and out from the
measuring chamber.
[0244] At step 2310, the ion concentration of the compound is
determined. The computing device evaluates a waveform that is in
accordance with the secondary pump current to determine the
concentration of one of the ions of the compound. The secondary
electrochemical cell system reduces the compound into the ion of
the element that is pumped into and out from the measuring chamber
and into another ion for which the concentration is to be
calculated. For NOx compounds, the PT/Rh electrode reduces the NOx
into N and O ions forming a local concentration of O ions near the
electrode. Accordingly, the pumped ion (O) has a first
concentration in a first region within the measuring chamber and a
second concentration within a second region within the measuring
chamber where the second region is closer to the electrode than the
first region. The local concentration of the ion is determined by
evaluating the control signal 1740 for controlling the second
switching current source. Any signal, however, that indicates the
duty cycle of the control signal (or secondary pump current) can be
evaluated to determine concentration. For example, the secondary
pump current may be directly evaluated in some cases. Also, the
output of the second comparator may be evaluated.
[0245] FIG. 24 is flow chart of a method of managing currents in
the sensor including a primary electrochemical cell system and a
secondary electrochemical cell system.
[0246] At step 2402, the primary pump current is directed in a
positive direction at a first constant magnitude until the first
output signal reaches an upper threshold. For the example, the
switching current source maintains a positive primary pump current
until the first comparator detects that the first output voltage
has reaches the upper threshold at which time the comparator output
causes (through the flip-flop) the switching current source to
change direction.
[0247] At step 2404, the primary pump current is directed in a
negative direction at the first constant magnitude until the first
output signal reaches a lower threshold. The switching current
source maintains a negative primary pump current until the first
comparator detects that the first output voltage has reaches the
lower threshold at which time the comparator output causes (through
the flip-flop) the switching current source to change direction
again and pump current in the positive direction and the first
magnitude. Steps 2402 and 2404 continue to repeat.
[0248] At step 2406, the secondary pump current is directed in a
positive direction at a second constant magnitude until a first
difference between the second output signal and the first output
signal reaches a first difference threshold. The second switching
current source maintains a positive secondary pump current until
the second comparator detects that the difference between the
second output signal and the first output voltage has reached a
difference threshold. Where the primary electrochemical cell system
includes a single electrochemical cell, the Nernst voltage portion
of the first signals is extracted and compared to the second output
voltage. When the first difference threshold is reached, the second
comparator output causes (through the second flip-flop) the second
switching current source to change direction.
[0249] At step 2408, the secondary pump current is directed in a
negative direction at the second constant magnitude until a second
difference between the second output signal and the first output
signal reaches a second difference threshold. The second switching
current source maintains a negative secondary pump current until
the second comparator detects that the difference between the
second output signal and the first output voltage has reaches a
second difference threshold. Where the primary electrochemical cell
system includes a single electrochemical cell, the Nernst voltage
portion of the first signals is extracted and compared to the
second output voltage. When the second difference threshold is
reached, the second comparator output causes (through the second
flip-flop) the second switching current source to change direction
again. The order of steps 2402, 2404, 2406, and 2408 may vary
during operation. For example, since the frequency of oscillation
of the primary pump current is much lower than the frequency of the
secondary pump current, steps 2406 and 2408 may repeat several
times before step 2404 is executed.
[0250] FIG. 25 is a block diagram of a sensor system 2500 including
a sealed chamber sensor 2502. The sealed chamber sensor 2502
includes an electrochemical measuring cell 2504 and an
electrochemical compensation cell 2506 where both cells 2504, 2506
are connected to a sealed chamber 2508. A measured fluid 2510 such
as measured gas or liquid, is received through a measuring opening
2512 into a measuring chamber 2514. The measuring opening 2512 may
be a diffusion gap, diffusion layer, or other membrane or orifice
that allows measured fluid 2508 to enter the measuring chamber
2514. The electrochemical measuring cell 2504 operates in
accordance with the description of the operation of the
electrochemical cell 1006 discussed above. The sealed chamber
sensor 2500, however, is less susceptible to adverse performance
resulting from contamination from dirt, water, and other
environmental elements. The electrochemical compensation cell 2506
insolates the electrochemical measuring cell 2504 from direct
exposure to these contaminates.
[0251] A measuring cell current 2516 is directed through the
electrochemical measuring cell 2504 to move ions between the sealed
chamber 2510 and the measuring chamber 2512. Accordingly, a first
ion flow 2518 corresponds to the measuring cell current 2516. A
compensation cell current 2520 directed though the electrochemical
compensation cell 2506 moves ions between the sealed chamber 2508
and an external fluid 2522 which may be ambient liquid, ambient
air, exhaust gas or other gas or liquid depending on the particular
implementation of the sensor 2500. The external fluid 2522 has an
adequate ion concentration to allow sufficient ion flow into the
sealed chamber 2510 from the external fluid 2522 through the
electrochemical compensation cell 2506. Accordingly, a second ion
flow 2524 corresponds to the compensation cell current 2520.
[0252] All ions entering and exiting the sealed air chamber 2510
enter and exit through one of the electrochemical cells 2504, 2506.
Accordingly, the sealed chamber 2510 is sealed in the sense that no
ions enter or exit the sealed chamber 2510 without passing through
one of the electrochemical cells 2504, 2506.
[0253] A sensor managing device 2526 includes any combination of
hardware, software and/or firmware for managing the currents 2516,
2520 and measuring the cell voltage (V.sub.CELL) 2528 across the
electrochemical measuring cell 2504 to determine an ion
concentration. The sensor managing device 2526, therefore, operates
as discussed above except that sensor managing device 2526 also
directs the appropriate compensation current 2520 through the
electrochemical compensation cell 2506. The compensation cell
current 2520 causes the ion flow 2524 that results in the same
volume of ions to flow out from sealed chamber 2510 through the
electrochemical compensation cell 2506 as the volume of ions
flowing into the sealed chamber 2510 through the electrochemical
measuring cell 2504 and the same number of ions to flow into the
sealed chamber 2510 through the electrochemical compensation cell
2506 as the volume of ions flowing out from the sealed chamber 2510
through the electrochemical measuring cell 2504. In certain
situations where the structure of the two cells 2504, 2506 is the
same, the currents 2516, 2520 have equal magnitudes but opposite
polarities. The sensor system 2500 may be implemented using any
combination of hardware, software and firmware. Various functions
and operations of the functional blocks described herein may be
implemented in any number of devices, circuits or elements. Any of
the functional blocks may be integrated in a single device and the
functions of the blocks may be distributed over several devices,
circuits and elements.
[0254] FIG. 26 is a block diagram of a cross section of an
electrochemical sensor 2600 including a sealed air chamber for
measuring exhaust gas. The sensor 2600, therefore, is an example of
the sensor 2502 of FIG. 25 where the measured fluid and external
fluid are gases. For the example of FIG. 26, the sensor 2600 is a
wideband single measuring cell sensor for measuring oxygen
concentration within the exhaust system of a combustion engine. The
sensor 2600 may also be used within other systems and for other
uses in some situations.
[0255] The sensor 2600 includes a single laminated zirconium
dioxide (ZrO.sub.2) ceramic layer 2604. A space between the
ZrO.sub.2 layer 2604 and a chamber housing 2606 forms the measuring
chamber 2514. A hole 2512 within the chamber housing 2606 forms a
diffusion gap for receiving a measured gas 2608. The measured
exhaust gas 2608, therefore, is an example of the measured fluid
2508. The hole 2512 may be a diffusion gap, diffusion layer, or
other opening into the measuring chamber 2514.
[0256] A single platinum electrode (air electrode) 2610 is disposed
on the ZrO.sub.2 layer 2604 such that the air electrode 2510 is
opposite the measuring chamber 2514 and exposed to sealed air 2612
within a sealed air chamber 2602. The sealed air chamber 2602 is an
example of the sealed chamber 2510.
[0257] A second platinum electrode (measuring electrode) 2614 is
disposed on the ZrO.sub.2 layer 2604 opposite the air electrode
2610 and is exposed to the measured gas 2608 within the measuring
chamber 2512 to form an oxygen electrochemical measuring cell 2616
with the air electrode 2610 and the ZrO.sub.2 layer 2604. A third
platinum electrode 2618 is disposed on the same side of ZrO.sub.2
layer 2604 as the second platinum electrode 2614 and is positioned
outside of the measuring chamber 2514 such that the third platinum
electrode 2618 is exposed to an external gas 2620 which may be
ambient air, exhaust gas or any other gas that includes adequate
volume of oxygen ions. The third platinum electrode 2618 is a
compensation electrode 2618 that forms a compensation
electrochemical cell 2622 with the air electrode 2610 and the
ZrO.sub.2 layer 2604. Accordingly, the measuring electrode 2614,
air electrode 2610, and ZrO.sub.2 layer 2604 form the
electrochemical oxygen measuring cell 2616 and the compensation
electrode 2618, air electrode 2610, and ZrO.sub.2 layer 2604 form
the compensation electrochemical cell 2522.
[0258] An electrical current (measuring cell current) through the
electrochemical oxygen measuring cell 2616 transports oxygen ions
in an opposite direction to the direction of the electrical
current. A compensation current with an opposite polarity to the
measuring cell current is directed through the compensation cell
which moves oxygen ions in a direction opposite the compensation
cell current. The currents are selected such that the volume of
oxygen ions pumped in to the measuring chamber by one of the cells
is equal to the volume of oxygen ions pumped out from the measuring
chamber by the other electrochemical cell. For example, where the
compensation electrode 2618 has the same surface area as the
measurement electrode 2614 and the electrochemical compensation
cell 2622 has similar structure to the electrochemical oxygen
measuring cell 2616, the compensation current is equal in magnitude
to the measurement current, but opposite in polarity.
[0259] As mentioned above, the external gas 2620 must have an
adequate volume of oxygen ions to allow the electrochemical
compensation cell 2622 to pump the same volume of oxygen ions
depleted from the sealed air chamber 2602 by the electrochemical
oxygen measuring cell 2616. Accordingly, for the example of FIG.
26, the external gas 2620 may be exhaust gas, air, or other gas
where there is adequate water, carbon dioxide, carbon monoxide or
other compounds containing sufficient oxygen ions to supply the ion
flow 2524.
[0260] The sealed air chamber 2602 ensures an adequate volume of
ambient gas to minimize or eliminate the possibility of
oxygen-depleted or oxygen-enriched gas being exposed to the air
electrode 2610. In sensors where a single cell is exposed to
ambient air through an opening, it may be possible for the opening
to become obstructed and air flow restricted. For example, off-road
vehicles are often exposes to dirt, dust, mud, and water and the
sensor may come in contact with these contaminants or sometimes be
submerged. Accordingly, an opening within a sensor opening may
become obstructed. A sensor with a sealed chamber, however, does
not require the air electrode to be exposed to ambient air
directly. The compensation cell provides an interface to the
outside air without using an opening.
[0261] Clearly, other embodiments and modifications of this
invention will occur readily to those of ordinary skill in the art
in view of these teachings. Therefore, this invention is to be
limited only by following claims, which include all such
embodiments, equivalents, and modifications when viewed in
conjunction with the above specification and accompanying
drawings.
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